path: root/lib/sqlalchemy/orm/unitofwork.py

# orm/unitofwork.py
# Copyright (C) 2005, 2006, 2007 Michael Bayer mike_mp@zzzcomputing.com
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""The internals for the Unit Of Work system.

Includes hooks into the attributes package enabling the routing of
change events to Unit Of Work objects, as well as the flush()
mechanism which creates a dependency structure that executes change
operations.

A Unit of Work is essentially a system of maintaining a graph of
in-memory objects and their modified state.  Objects are maintained as
unique against their primary key identity using an *identity map*
pattern.  The Unit of Work then maintains lists of objects that are
new, dirty, or deleted and provides the capability to flush all those
changes at once.
"""

from sqlalchemy import util, logging, topological, exceptions
from sqlalchemy.orm import attributes, interfaces
from sqlalchemy.orm import util as mapperutil
from sqlalchemy.orm.mapper import object_mapper
import StringIO
import weakref

# Load lazily
object_session = None

class UOWEventHandler(interfaces.AttributeExtension):
    """An event handler added to all class attributes which handles
    session operations.
    """

    def __init__(self, key, class_, cascade=None):
        self.key = key
        self.class_ = class_
        self.cascade = cascade

    def append(self, obj, item, initiator):
        # process "save_update" cascade rules for when an instance is appended to the list of another instance
        sess = object_session(obj)
        if sess is not None:
            if self.cascade is not None and self.cascade.save_update and item not in sess:
                mapper = object_mapper(obj)
                prop = mapper.get_property(self.key)
                ename = prop.mapper.entity_name
                sess.save_or_update(item, entity_name=ename)

    def remove(self, obj, item, initiator):
        # currently no cascade rules for removing an item from a list
        # (i.e. it stays in the Session)
        pass

    def set(self, obj, newvalue, oldvalue, initiator):
        # process "save_update" cascade rules for when an instance is attached to another instance
        sess = object_session(obj)
        if sess is not None:
            if newvalue is not None and self.cascade is not None and self.cascade.save_update and newvalue not in sess:
                mapper = object_mapper(obj)
                prop = mapper.get_property(self.key)
                ename = prop.mapper.entity_name
                sess.save_or_update(newvalue, entity_name=ename)


class UOWAttributeManager(attributes.AttributeManager):
    """Override ``AttributeManager`` to provide the ``UOWProperty``
    instance for all ``InstrumentedAttributes``.
    """

    def create_prop(self, class_, key, uselist, callable_, typecallable,
                    cascade=None, extension=None, **kwargs):
        extension = util.to_list(extension or [])
        extension.insert(0, UOWEventHandler(key, class_, cascade=cascade))

        return super(UOWAttributeManager, self).create_prop(
            class_, key, uselist, callable_, typecallable,
            extension=extension, **kwargs)


class UnitOfWork(object):
    """Main UOW object which stores lists of dirty/new/deleted objects.

    Provides top-level *flush* functionality as well as the
    default transaction boundaries involved in a write
    operation.
    """

    def __init__(self, identity_map=None, weak_identity_map=False):
        if identity_map is not None:
            self.identity_map = identity_map
        else:
            if weak_identity_map:
                self.identity_map = weakref.WeakValueDictionary()
            else:
                self.identity_map = {}

        self.new = util.Set() #OrderedSet()
        self.deleted = util.Set()
        self.logger = logging.instance_logger(self)

    echo = logging.echo_property()

    def _remove_deleted(self, obj):
        if hasattr(obj, "_instance_key"):
            del self.identity_map[obj._instance_key]
        try:
            self.deleted.remove(obj)
        except KeyError:
            pass
        try:
            self.new.remove(obj)
        except KeyError:
            pass

    def _validate_obj(self, obj):
        if (hasattr(obj, '_instance_key') and not self.identity_map.has_key(obj._instance_key)) or \
            (not hasattr(obj, '_instance_key') and obj not in self.new):
            raise exceptions.InvalidRequestError("Instance '%s' is not attached or pending within this session" % repr(obj))

    def _is_valid(self, obj):
        if (hasattr(obj, '_instance_key') and not self.identity_map.has_key(obj._instance_key)) or \
            (not hasattr(obj, '_instance_key') and obj not in self.new):
            return False
        else:
            return True

    def register_clean(self, obj):
        """register the given object as 'clean' (i.e. persistent) within this unit of work."""
        
        if obj in self.new:
            self.new.remove(obj)
        if not hasattr(obj, '_instance_key'):
            mapper = object_mapper(obj)
            obj._instance_key = mapper.identity_key_from_instance(obj)
        if hasattr(obj, '_sa_insert_order'):
            delattr(obj, '_sa_insert_order')
        self.identity_map[obj._instance_key] = obj
        attribute_manager.commit(obj)

    def register_new(self, obj):
        """register the given object as 'new' (i.e. unsaved) within this unit of work."""

        if hasattr(obj, '_instance_key'):
            raise exceptions.InvalidRequestError("Object '%s' already has an identity - it can't be registered as new" % repr(obj))
        if obj not in self.new:
            self.new.add(obj)
            obj._sa_insert_order = len(self.new)

    def register_deleted(self, obj):
        """register the given persistent object as 'to be deleted' within this unit of work."""
        
        if obj not in self.deleted:
            self._validate_obj(obj)
            self.deleted.add(obj)

    def locate_dirty(self):
        """return a set of all persistent instances within this unit of work which 
        either contain changes or are marked as deleted.
        """
        
        return util.Set([x for x in self.identity_map.values() if x not in self.deleted and attribute_manager.is_modified(x)])

    def flush(self, session, objects=None):
        """create a dependency tree of all pending SQL operations within this unit of work and execute."""
        
        # this context will track all the objects we want to save/update/delete,
        # and organize a hierarchical dependency structure.  it also handles
        # communication with the mappers and relationships to fire off SQL
        # and synchronize attributes between related objects.
        echo = logging.is_info_enabled(self.logger)

        flush_context = UOWTransaction(self, session)

        # create the set of all objects we want to operate upon
        if objects is not None:
            # specific list passed in
            objset = util.Set(objects)
        else:
            # or just everything
            objset = util.Set(self.identity_map.values()).union(self.new)

        # detect persistent objects that have changes
        dirty = self.locate_dirty()

        # store objects whose fate has been decided
        processed = util.Set()

        # put all saves/updates into the flush context.  detect top-level orphans and throw them into deleted.
        for obj in self.new.union(dirty).intersection(objset).difference(self.deleted):
            if obj in processed:
                continue

            flush_context.register_object(obj, isdelete=object_mapper(obj)._is_orphan(obj))
            processed.add(obj)

        # put all remaining deletes into the flush context.
        for obj in self.deleted.intersection(objset).difference(processed):
            flush_context.register_object(obj, isdelete=True)

        session.create_transaction(autoflush=False)
        flush_context.transaction = session.transaction
        try:
            flush_context.execute()
        except:
            session.rollback()
            raise
        session.commit()

        flush_context.post_exec()

class UOWTransaction(object):
    """Handles the details of organizing and executing transaction
    tasks during a UnitOfWork object's flush() operation.  
    
    The central operation is to form a graph of nodes represented by the
    ``UOWTask`` class, which is then traversed by a ``UOWExecutor`` object
    that issues SQL and instance-synchronizing operations via the related
    packages.
    """

    def __init__(self, uow, session):
        self.uow = uow
        self.session = session
        self.mapper_flush_opts = session._mapper_flush_opts
        
        # stores tuples of mapper/dependent mapper pairs,
        # representing a partial ordering fed into topological sort
        self.dependencies = util.Set()
        
        # dictionary of mappers to UOWTasks
        self.tasks = {}
        
        # dictionary used by external actors to store arbitrary state
        # information. 
        self.attributes = {}

        self.logger = logging.instance_logger(self)
        self.echo = uow.echo
        
    echo = logging.echo_property()

    def register_object(self, obj, isdelete = False, listonly = False, postupdate=False, post_update_cols=None, **kwargs):
        """Add an object to this ``UOWTransaction`` to be updated in the database.

        This operation has the combined effect of locating/creating an appropriate
        ``UOWTask`` object, and calling its ``append()`` method which then locates/creates
        an appropriate ``UOWTaskElement`` object.
        """

        #print "REGISTER", repr(obj), repr(getattr(obj, '_instance_key', None)), str(isdelete), str(listonly)

        # if object is not in the overall session, do nothing
        if not self.uow._is_valid(obj):
            if logging.is_debug_enabled(self.logger):
                self.logger.debug("object %s not part of session, not registering for flush" % (mapperutil.instance_str(obj)))
            return

        if logging.is_debug_enabled(self.logger):
            self.logger.debug("register object for flush: %s isdelete=%s listonly=%s postupdate=%s" % (mapperutil.instance_str(obj), isdelete, listonly, postupdate))

        mapper = object_mapper(obj)
        task = self.get_task_by_mapper(mapper)
        if postupdate:
            task.append_postupdate(obj, post_update_cols)
            return

        task.append(obj, listonly, isdelete=isdelete, **kwargs)

    def unregister_object(self, obj):
        """remove an object from its parent UOWTask.
        
        called by mapper.save_obj() when an 'identity switch' is detected, so that
        no further operations occur upon the instance."""
        mapper = object_mapper(obj)
        task = self.get_task_by_mapper(mapper)
        if obj in task._objects:
            task.delete(obj)

    def is_deleted(self, obj):
        """return true if the given object is marked as deleted within this UOWTransaction."""
        
        mapper = object_mapper(obj)
        task = self.get_task_by_mapper(mapper)
        return task.is_deleted(obj)

    def get_task_by_mapper(self, mapper, dontcreate=False):
        """return UOWTask element corresponding to the given mapper.

        Will create a new UOWTask, including a UOWTask corresponding to the 
        "base" inherited mapper, if needed, unless the dontcreate flag is True.
        """
        try:
            return self.tasks[mapper]
        except KeyError:
            if dontcreate:
                return None
                
            base_mapper = mapper.base_mapper()
            if base_mapper in self.tasks:
                base_task = self.tasks[base_mapper]
            else:
                base_task = UOWTask(self, base_mapper)
                self.tasks[base_mapper] = base_task
                base_mapper.register_dependencies(self)

            if mapper not in self.tasks:
                task = UOWTask(self, mapper, base_task=base_task)
                self.tasks[mapper] = task
                mapper.register_dependencies(self)
            else:
                task = self.tasks[mapper]
                
            return task

    def register_dependency(self, mapper, dependency):
        """register a dependency between two mappers.

        Called by ``mapper.PropertyLoader`` to register the objects
        handled by one mapper being dependent on the objects handled
        by another.        
        """

        # correct for primary mapper (the mapper offcially associated with the class)
        # also convert to the "base mapper", the parentmost task at the top of an inheritance chain
        # dependency sorting is done via non-inheriting mappers only, dependencies between mappers
        # in the same inheritance chain is done at the per-object level
        mapper = mapper.primary_mapper().base_mapper()
        dependency = dependency.primary_mapper().base_mapper()

        self.dependencies.add((mapper, dependency))

    def register_processor(self, mapper, processor, mapperfrom):
        """register a dependency processor object, corresponding to dependencies between
        the two given mappers.
        
        In reality, the processor is an instance of ``dependency.DependencyProcessor``
        and is registered as a result of the ``mapper.register_dependencies()`` call in
        ``get_task_by_mapper()``.
        
        The dependency processor supports the methods ``preprocess_dependencies()`` and
        ``process_dependencies()``, which
        perform operations on a list of instances that have a dependency relationship
        with some other instance.  The operations include adding items to the UOW
        corresponding to some cascade operations, issuing inserts/deletes on 
        association tables, and synchronzing foreign key values between related objects
        before the dependent object is operated upon at the SQL level.
        """

        # when the task from "mapper" executes, take the objects from the task corresponding
        # to "mapperfrom"'s list of save/delete objects, and send them to "processor"
        # for dependency processing

        #print "registerprocessor", str(mapper), repr(processor), repr(processor.key), str(mapperfrom)

        # correct for primary mapper (the mapper offcially associated with the class)
        mapper = mapper.primary_mapper()
        mapperfrom = mapperfrom.primary_mapper()

        task = self.get_task_by_mapper(mapper)
        targettask = self.get_task_by_mapper(mapperfrom)
        up = UOWDependencyProcessor(processor, targettask)
        task._dependencies.add(up)
        
    def execute(self):
        """Execute this UOWTransaction.
        
        This will organize all collected UOWTasks into a toplogically-sorted
        dependency tree, which is then traversed using the traversal scheme
        encoded in the UOWExecutor class.  Operations to mappers and dependency
        processors are fired off in order to issue SQL to the database and 
        to maintain instance state during the execution."""

        # pre-execute dependency processors.  this process may
        # result in new tasks, objects and/or dependency processors being added,
        # particularly with 'delete-orphan' cascade rules.
        # keep running through the full list of tasks until all
        # objects have been processed.
        while True:
            ret = False
            for task in self.tasks.values():
                for up in list(task.dependencies):
                    if up.preexecute(self):
                        ret = True
            if not ret:
                break

        head = self._sort_dependencies()
        if self.echo:
            if head is None:
                self.logger.info("Task dump: None")
            else:
                self.logger.info("Task dump:\n" + head.dump())
        if head is not None:
            UOWExecutor().execute(self, head)
        self.logger.info("Execute Complete")

    def post_exec(self):
        """mark processed objects as clean / deleted after a successful flush().
        
        this method is called within the flush() method after the 
        execute() method has succeeded and the transaction has been committed.
        """

        for task in self.tasks.values():
            for elem in task.elements:
                if elem.obj is None:
                    continue
                if elem.isdelete:
                    self.uow._remove_deleted(elem.obj)
                else:
                    self.uow.register_clean(elem.obj)

    def _sort_dependencies(self):
        """Create a hierarchical tree of dependent UOWTask instances.

        The root UOWTask is returned.  
        
        Cyclical relationships
        within the toplogical sort are further broken down into new
        temporary UOWTask insances which represent smaller sub-groups of objects
        that would normally belong to a single UOWTask.

        """

        def sort_hier(node):
            if node is None:
                return None
            task = self.get_task_by_mapper(node.item)
            if node.cycles is not None:
                tasks = []
                for n in node.cycles:
                    tasks.append(self.get_task_by_mapper(n.item))
                task = task._sort_circular_dependencies(self, tasks)
            for child in node.children:
                t = sort_hier(child)
                if t is not None:
                    task.childtasks.append(t)
            return task

        # get list of base mappers
        mappers = [t.mapper for t in self.tasks.values() if t.base_task is t]
        head = topological.QueueDependencySorter(self.dependencies, mappers).sort(allow_all_cycles=True)
        if logging.is_debug_enabled(self.logger):
            self.logger.debug("Dependent tuples:\n" + "\n".join(["(%s->%s)" % (d[0].class_.__name__, d[1].class_.__name__) for d in self.dependencies]))
            self.logger.debug("Dependency sort:\n"+ str(head))
        task = sort_hier(head)
        return task


class UOWTask(object):
    """Represents all of the objects in the UOWTransaction which correspond to
    a particular mapper.  This is the primary class of three classes used to generate
    the elements of the dependency graph.
    """

    def __init__(self, uowtransaction, mapper, base_task=None):
        # the transaction owning this UOWTask
        self.uowtransaction = uowtransaction

        # base_task is the UOWTask which represents the "base mapper"
        # in our mapper's inheritance chain.  if the mapper does not
        # inherit from any other mapper, the base_task is self.
        # the _inheriting_tasks dictionary is a dictionary present only
        # on the "base_task"-holding UOWTask, which maps all mappers within
        # an inheritance hierarchy to their corresponding UOWTask instances.
        if base_task is None:
            self.base_task = self
            self._inheriting_tasks = {mapper:self}
        else:
            self.base_task = base_task
            base_task._inheriting_tasks[mapper] = self
        
        # the Mapper which this UOWTask corresponds to
        self.mapper = mapper

        # a dictionary mapping object instances to a corresponding UOWTaskElement.
        # Each UOWTaskElement represents one object instance which is to be saved or
        # deleted by this UOWTask's Mapper.
        # in the case of the row-based "cyclical sort", the UOWTaskElement may
        # also reference further UOWTasks which are dependent on that UOWTaskElement.
        self._objects = {} 

        # a set of UOWDependencyProcessor instances, which are executed after saves and
        # before deletes, to synchronize data between dependent objects as well as to
        # ensure that relationship cascades populate the flush() process with all
        # appropriate objects.
        self._dependencies = util.Set()

        # a list of UOWTasks which are sub-nodes to this UOWTask.  this list 
        # is populated during the dependency sorting operation.
        self.childtasks = []
        
        # a list of UOWDependencyProcessor instances
        # which derive from the UOWDependencyProcessor instances present in a
        # corresponding UOWTask's "_dependencies" set.  This collection is populated
        # during a row-based cyclical sorting operation and only corresponds to 
        # new UOWTask instances created during this operation, which are also local
        # to the dependency graph (i.e. they are not present in the get_task_by_mapper()
        # collection).
        self._cyclical_dependencies = util.Set()

    def polymorphic_tasks(self):
        """return an iterator of UOWTask objects corresponding to the inheritance sequence
        of this UOWTask's mapper.
        
            e.g. if mapper B and mapper C inherit from mapper A, and mapper D inherits from B:
            
                mapperA -> mapperB -> mapperD
                        -> mapperC 
                                   
            the inheritance sequence starting at mapper A is a depth-first traversal:
            
                [mapperA, mapperB, mapperD, mapperC]
                
            this method will therefore return
            
                [UOWTask(mapperA), UOWTask(mapperB), UOWTask(mapperD), UOWTask(mapperC)]
                
        The concept of "polymporphic iteration" is adapted into several property-based 
        iterators which return object instances, UOWTaskElements and UOWDependencyProcessors
        in an order corresponding to this sequence of parent UOWTasks.  This is used to issue
        operations related to inheritance-chains of mappers in the proper order based on 
        dependencies between those mappers.
        
        """
        
        for mapper in self.mapper.polymorphic_iterator():
            t = self.base_task._inheriting_tasks.get(mapper, None)
            if t is not None:
                yield t
            
    def is_empty(self):
        """return True if this UOWTask is 'empty', meaning it has no child items.
        
        used only for debugging output.
        """
        
        return len(self._objects) == 0 and len(self._dependencies) == 0 and len(self.childtasks) == 0

    def append(self, obj, listonly = False, childtask = None, isdelete = False):
        """Append an object to this task to be persisted or deleted.
        
        The actual record added to the ``UOWTask`` is a ``UOWTaskElement`` object
        corresponding to the given instance.  If a corresponding ``UOWTaskElement`` already
        exists within this ``UOWTask``, its state is updated with the given 
        keyword arguments as appropriate.
        
        'isdelete' when True indicates the operation will be a "delete" 
        operation (i.e. DELETE), otherwise is a "save" operation (i.e. INSERT/UPDATE).
        a ``UOWTaskElement`` marked as "save" which receives the "isdelete" flag will 
        be marked as deleted, but the reverse operation does not apply (i.e. goes from
        "delete" to being "not delete").

        `listonly` indicates that the object does not require a delete
        or save operation, but does require dependency operations to be 
        executed.  For example, adding a child object to a parent via a
        one-to-many relationship requires that a ``OneToManyDP`` object
        corresponding to the parent's mapper synchronize the instance's primary key
        value into the foreign key attribute of the child object, even though 
        no changes need be persisted on the parent.
        
        a listonly object may be "upgraded" to require a save/delete operation
        by a subsequent append() of the same object instance with the `listonly`
        flag set to False.  once the flag is set to false, it stays that way
        on the ``UOWTaskElement``.
        
        `childtask` is an optional ``UOWTask`` element represending operations which
        are dependent on the parent ``UOWTaskElement``.  This flag is only used on 
        `UOWTask` objects created within the "cyclical sort" part of the hierarchical
        sort, which generates a dependency tree of individual instances instead of 
        mappers when cycles between mappers are detected.
        """

        try:
            rec = self._objects[obj]
            retval = False
        except KeyError:
            rec = UOWTaskElement(obj)
            self._objects[obj] = rec
            retval = True
        if not listonly:
            rec.listonly = False
        if childtask:
            rec.childtasks.append(childtask)
        if isdelete:
            rec.isdelete = True
        return retval

    def append_postupdate(self, obj, post_update_cols):
        """issue a 'post update' UPDATE statement via this object's mapper immediately.  
        
        this operation is used only with relations that specify the `post_update=True`
        flag.
        """

        # postupdates are UPDATED immeditely (for now)
        # convert post_update_cols list to a Set so that __hashcode__ is used to compare columns
        # instead of __eq__
        self.mapper.save_obj([obj], self.uowtransaction, postupdate=True, post_update_cols=util.Set(post_update_cols))
        return True

    def delete(self, obj):
        """remove the given object from this UOWTask, if present."""
        
        try:
            del self._objects[obj]
        except KeyError:
            pass

    def __contains__(self, obj):
        """return True if the given object is contained within this UOWTask or inheriting tasks."""
        
        for task in self.polymorphic_tasks():
            if obj in task._objects:
                return True
        else:
            return False

    def is_deleted(self, obj):
        """return True if the given object is marked as to be deleted within this UOWTask."""
        
        try:
            return self._objects[obj].isdelete
        except KeyError:
            return False

    def _polymorphic_collection(callable):
        """return a property that will adapt the collection returned by the
        given callable into a polymorphic traversal."""
        
        def collection(self):
            for task in self.polymorphic_tasks():
                for rec in callable(task):
                    yield rec
        return property(collection)
        
    elements = property(lambda self:self._objects.values())
    
    polymorphic_elements = _polymorphic_collection(lambda task:task.elements)

    polymorphic_tosave_elements = property(lambda self: [rec for rec in self.polymorphic_elements
                                             if not rec.isdelete])
                                             
    polymorphic_todelete_elements = property(lambda self:[rec for rec in self.polymorphic_elements
                                               if rec.isdelete])

    polymorphic_tosave_objects = property(lambda self:[rec.obj for rec in self.polymorphic_elements
                                          if rec.obj is not None and not rec.listonly and rec.isdelete is False])

    polymorphic_todelete_objects = property(lambda self:[rec.obj for rec in self.polymorphic_elements
                                          if rec.obj is not None and not rec.listonly and rec.isdelete is True])

    dependencies = property(lambda self:self._dependencies)
    
    cyclical_dependencies = property(lambda self:self._cyclical_dependencies)
    
    polymorphic_dependencies = _polymorphic_collection(lambda task:task.dependencies)
    
    polymorphic_childtasks = _polymorphic_collection(lambda task:task.childtasks)
    
    polymorphic_cyclical_dependencies = _polymorphic_collection(lambda task:task.cyclical_dependencies)
    
    def _sort_circular_dependencies(self, trans, cycles):
        """Create a hierarchical tree of *subtasks*
        which associate specific dependency actions with individual
        objects.  This is used for a *cyclical* task, or a task where
        elements of its object list contain dependencies on each
        other.

        This is not the normal case; this logic only kicks in when
        something like a hierarchical tree is being represented.
        """
        allobjects = []
        for task in cycles:
            allobjects += [e.obj for e in task.polymorphic_elements]
        tuples = []

        cycles = util.Set(cycles)

        #print "BEGIN CIRC SORT-------"
        #print "PRE-CIRC:"
        #print list(cycles) #[0].dump()

        # dependency processors that arent part of the cyclical thing
        # get put here
        extradeplist = []

        # organizes a set of new UOWTasks that will be assembled into
        # the final tree, for the purposes of holding new UOWDependencyProcessors
        # which process small sub-sections of dependent parent/child operations
        dependencies = {}

        def get_dependency_task(obj, depprocessor):
            try:
                dp = dependencies[obj]
            except KeyError:
                dp = dependencies.setdefault(obj, {})
            try:
                l = dp[depprocessor]
            except KeyError:
                l = UOWTask(self.uowtransaction, depprocessor.targettask.mapper)
                dp[depprocessor] = l
            return l

        def dependency_in_cycles(dep):
            proctask = trans.get_task_by_mapper(dep.processor.mapper.base_mapper(), True)
            targettask = trans.get_task_by_mapper(dep.targettask.mapper.base_mapper(), True)
            return targettask in cycles and (proctask is not None and proctask in cycles)

        # organize all original UOWDependencyProcessors by their target task
        deps_by_targettask = {}
        for task in cycles:
            for dep in task.polymorphic_dependencies:
                if not dependency_in_cycles(dep):
                    extradeplist.append(dep)
                for t in dep.targettask.polymorphic_tasks():
                    l = deps_by_targettask.setdefault(t, [])
                    l.append(dep)

        object_to_original_task = {}

        for task in cycles:
            for subtask in task.polymorphic_tasks():
                for taskelement in subtask.elements:
                    obj = taskelement.obj
                    object_to_original_task[obj] = subtask
                    for dep in deps_by_targettask.get(subtask, []):
                        # is this dependency involved in one of the cycles ?
                        if not dependency_in_cycles(dep):
                            continue
                        (processor, targettask) = (dep.processor, dep.targettask)
                        isdelete = taskelement.isdelete

                        # list of dependent objects from this object
                        childlist = dep.get_object_dependencies(obj, trans, passive=True)
                        if childlist is None:
                            continue
                        # the task corresponding to saving/deleting of those dependent objects
                        childtask = trans.get_task_by_mapper(processor.mapper)

                        childlist = childlist.added_items() + childlist.unchanged_items() + childlist.deleted_items()

                        for o in childlist:
                            # other object is None.  this can occur if the relationship is many-to-one
                            # or one-to-one, and None was set.  the "removed" object will be picked
                            # up in this iteration via the deleted_items() part of the collection.
                            if o is None:
                                continue

                            # the other object is not in the UOWTransaction !  but if we are many-to-one,
                            # we need a task in order to attach dependency operations, so establish a "listonly"
                            # task
                            if o not in childtask:
                                childtask.append(o, listonly=True)
                                object_to_original_task[o] = childtask

                            # create a tuple representing the "parent/child"
                            whosdep = dep.whose_dependent_on_who(obj, o)
                            if whosdep is not None:
                                # append the tuple to the partial ordering.
                                tuples.append(whosdep)

                                # create a UOWDependencyProcessor representing this pair of objects.
                                # append it to a UOWTask
                                if whosdep[0] is obj:
                                    get_dependency_task(whosdep[0], dep).append(whosdep[0], isdelete=isdelete)
                                else:
                                    get_dependency_task(whosdep[0], dep).append(whosdep[1], isdelete=isdelete)
                            else:
                                get_dependency_task(obj, dep).append(obj, isdelete=isdelete)

        #print "TUPLES", tuples
        #print "ALLOBJECTS", allobjects
        head = topological.QueueDependencySorter(tuples, allobjects).sort()
        
        # create a tree of UOWTasks corresponding to the tree of object instances
        # created by the DependencySorter
        
        used_tasks = util.Set()
        def make_task_tree(node, parenttask, nexttasks):
            originating_task = object_to_original_task[node.item]
            used_tasks.add(originating_task)
            t = nexttasks.get(originating_task, None)
            if t is None:
                t = UOWTask(self.uowtransaction, originating_task.mapper)
                nexttasks[originating_task] = t
                parenttask.append(None, listonly=False, isdelete=originating_task._objects[node.item].isdelete, childtask=t)
            t.append(node.item, originating_task._objects[node.item].listonly, isdelete=originating_task._objects[node.item].isdelete)

            if dependencies.has_key(node.item):
                for depprocessor, deptask in dependencies[node.item].iteritems():
                    t.cyclical_dependencies.add(depprocessor.branch(deptask))
            nd = {}
            for n in node.children:
                t2 = make_task_tree(n, t, nd)
            return t

        t = UOWTask(self.uowtransaction, self.mapper)
        
        # stick the non-circular dependencies onto the new UOWTask
        for d in extradeplist:
            t._dependencies.add(d)
        
        # if we have a head from the dependency sort, assemble child nodes
        # onto the tree.  note this only occurs if there were actual objects
        # to be saved/deleted.
        if head is not None:
            make_task_tree(head, t, {})

        for t2 in cycles:
            # tasks that were in the cycle but did not get assembled
            # into the tree, add them as child tasks.  these tasks
            # will have no "save" or "delete" members, but may have dependency
            # processors that operate upon other tasks outside of the cycle.
            if t2 not in used_tasks and t2 is not self:
                # the task must be copied into a "cyclical" task, so that polymorphic
                # rules dont fire off.  this ensures that the task will have no "save"
                # or "delete" members due to inheriting mappers which contain tasks
                localtask = UOWTask(self.uowtransaction, t2.mapper)
                for obj in t2.elements:
                    localtask.append(obj, t2.listonly, isdelete=t2._objects[obj].isdelete)
                for dep in t2.dependencies:
                    localtask._dependencies.add(dep)
                t.childtasks.insert(0, localtask)
        
        return t

    def dump(self):
        """return a string representation of this UOWTask and its 
        full dependency graph."""
        
        buf = StringIO.StringIO()
        import uowdumper
        uowdumper.UOWDumper(self, buf)
        return buf.getvalue()

    def __repr__(self):
        if self.mapper is not None:
            if self.mapper.__class__.__name__ == 'Mapper':
                name = self.mapper.class_.__name__ + "/" + self.mapper.local_table.name
            else:
                name = repr(self.mapper)
        else:
            name = '(none)'
        return ("UOWTask(%s) Mapper: '%s'" % (hex(id(self)), name))

class UOWTaskElement(object):
    """An element within a UOWTask.

    Corresponds to a single object instance to be saved, deleted, or
    just part of the transaction as a placeholder for further
    dependencies (i.e. 'listonly').

    In the case of a ``UOWTaskElement`` present within an instance-level
    graph formed due to cycles within the mapper-level graph, may also store a list of
    childtasks, further UOWTasks containing objects dependent on this
    element's object instance.
    """

    def __init__(self, obj):
        self.obj = obj
        self.__listonly = True
        self.childtasks = []
        self.__isdelete = False
        self.__preprocessed = {}

    def _get_listonly(self):
        return self.__listonly

    def _set_listonly(self, value):
        """Set_listonly is a one-way setter, will only go from True to False."""

        if not value and self.__listonly:
            self.__listonly = False
            self.clear_preprocessed()

    def _get_isdelete(self):
        return self.__isdelete

    def _set_isdelete(self, value):
        if self.__isdelete is not value:
            self.__isdelete = value
            self.clear_preprocessed()

    listonly = property(_get_listonly, _set_listonly)
    isdelete = property(_get_isdelete, _set_isdelete)

    def mark_preprocessed(self, processor):
        """Mark this element as *preprocessed* by a particular ``UOWDependencyProcessor``.

        Preprocessing is the step which sweeps through all the
        relationships on all the objects in the flush transaction and
        adds other objects which are also affected.  The actual logic is
        part of ``UOWTransaction.execute()``.
        
        The preprocessing operations
        are determined in part by the cascade rules indicated on a relationship,
        and in part based on the normal semantics of relationships.
        In some cases it can switch an object's state from *tosave* to *todelete*.

        Changes to the state of this ``UOWTaskElement`` will reset all
        *preprocessed* flags, causing it to be preprocessed again.
        When all ``UOWTaskElements have been fully preprocessed by all
        UOWDependencyProcessors, then the topological sort can be
        done.
        """

        self.__preprocessed[processor] = True

    def is_preprocessed(self, processor):
        return self.__preprocessed.get(processor, False)

    def clear_preprocessed(self):
        self.__preprocessed.clear()

    def __repr__(self):
        return "UOWTaskElement/%d: %s/%d %s" % (id(self), self.obj.__class__.__name__, id(self.obj), (self.listonly and 'listonly' or (self.isdelete and 'delete' or 'save')) )

class UOWDependencyProcessor(object):
    """In between the saving and deleting of objects, process
    *dependent* data, such as filling in a foreign key on a child item
    from a new primary key, or deleting association rows before a
    delete.  This object acts as a proxy to a DependencyProcessor.
    """

    def __init__(self, processor, targettask):
        self.processor = processor
        self.targettask = targettask

    def __repr__(self):
        return "UOWDependencyProcessor(%s, %s)" % (str(self.processor), str(self.targettask))
    
    def __str__(self):
        return repr(self)
            
    def __eq__(self, other):
        return other.processor is self.processor and other.targettask is self.targettask

    def __hash__(self):
        return hash((self.processor, self.targettask))

    def preexecute(self, trans):
        """preprocess all objects contained within this ``UOWDependencyProcessor``s target task.

        This may locate additional objects which should be part of the
        transaction, such as those affected deletes, orphans to be
        deleted, etc.
        
        Once an object is preprocessed, its ``UOWTaskElement`` is marked as processed.  If subsequent 
        changes occur to the ``UOWTaskElement``, its processed flag is reset, and will require processing
        again.

        Return True if any objects were preprocessed, or False if no
        objects were preprocessed.  If True is returned, the parent ``UOWTransaction`` will
        ultimately call ``preexecute()`` again on all processors until no new objects are processed.
        """

        def getobj(elem):
            elem.mark_preprocessed(self)
            return elem.obj

        ret = False
        elements = [getobj(elem) for elem in self.targettask.polymorphic_tosave_elements if elem.obj is not None and not elem.is_preprocessed(self)]
        if len(elements):
            ret = True
            self.processor.preprocess_dependencies(self.targettask, elements, trans, delete=False)

        elements = [getobj(elem) for elem in self.targettask.polymorphic_todelete_elements if elem.obj is not None and not elem.is_preprocessed(self)]
        if len(elements):
            ret = True
            self.processor.preprocess_dependencies(self.targettask, elements, trans, delete=True)
        return ret

    def execute(self, trans, delete):
        """process all objects contained within this ``UOWDependencyProcessor``s target task."""
        
        if not delete:
            self.processor.process_dependencies(self.targettask, [elem.obj for elem in self.targettask.polymorphic_tosave_elements if elem.obj is not None], trans, delete=False)
        else:
            self.processor.process_dependencies(self.targettask, [elem.obj for elem in self.targettask.polymorphic_todelete_elements if elem.obj is not None], trans, delete=True)

    def get_object_dependencies(self, obj, trans, passive):
        return self.processor.get_object_dependencies(obj, trans, passive=passive)

    def whose_dependent_on_who(self, obj, o):
        """establish which object is operationally dependent amongst a parent/child 
        using the semantics stated by the dependency processor.
        
        This method is used to establish a partial ordering (set of dependency tuples)
        when toplogically sorting on a per-instance basis.
        
        """
        
        return self.processor.whose_dependent_on_who(obj, o)

    def branch(self, task):
        """create a copy of this ``UOWDependencyProcessor`` against a new ``UOWTask`` object.
        
        this is used within the instance-level sorting operation when a single ``UOWTask``
        is broken up into many individual ``UOWTask`` objects.
        
        """
        
        return UOWDependencyProcessor(self.processor, task)
    
        
class UOWExecutor(object):
    """Encapsulates the execution traversal of a UOWTransaction structure."""

    def execute(self, trans, task, isdelete=None):
        if isdelete is not True:
            self.execute_save_steps(trans, task)
        if isdelete is not False:
            self.execute_delete_steps(trans, task)

    def save_objects(self, trans, task):
        task.mapper.save_obj(task.polymorphic_tosave_objects, trans)

    def delete_objects(self, trans, task):
        task.mapper.delete_obj(task.polymorphic_todelete_objects, trans)

    def execute_dependency(self, trans, dep, isdelete):
        dep.execute(trans, isdelete)

    def execute_save_steps(self, trans, task):
        self.save_objects(trans, task)
        self.execute_cyclical_dependencies(trans, task, False)
        self.execute_per_element_childtasks(trans, task, False)
        self.execute_dependencies(trans, task, False)
        self.execute_dependencies(trans, task, True)
        self.execute_childtasks(trans, task, False)

    def execute_delete_steps(self, trans, task):
        self.execute_cyclical_dependencies(trans, task, True)
        self.execute_childtasks(trans, task, True)
        self.execute_per_element_childtasks(trans, task, True)
        self.delete_objects(trans, task)

    def execute_dependencies(self, trans, task, isdelete=None):
        if isdelete is not True:
            for dep in task.polymorphic_dependencies:
                self.execute_dependency(trans, dep, False)
        if isdelete is not False:
            for dep in util.reversed(list(task.polymorphic_dependencies)):
                self.execute_dependency(trans, dep, True)

    def execute_childtasks(self, trans, task, isdelete=None):
        for child in task.polymorphic_childtasks:
            self.execute(trans, child, isdelete)

    def execute_cyclical_dependencies(self, trans, task, isdelete):
        for dep in task.polymorphic_cyclical_dependencies:
            self.execute_dependency(trans, dep, isdelete)

    def execute_per_element_childtasks(self, trans, task, isdelete):
        for element in task.polymorphic_tosave_elements + task.polymorphic_todelete_elements:
            self.execute_element_childtasks(trans, element, isdelete)

    def execute_element_childtasks(self, trans, element, isdelete):
        for child in element.childtasks:
            self.execute(trans, child, isdelete)

# the AttributeManager used by the UOW/Session system to instrument
# object instances and track history.
attribute_manager = UOWAttributeManager()