Add anonymizing context to cache keys, comparison; convert traversal

Created new visitor system called "internal traversal" that
applies a data driven approach to the concept of a class that
defines its own traversal steps, in contrast to the existing
style of traversal now known as "external traversal" where
the visitor class defines the traversal, i.e. the SQLCompiler.

The internal traversal system now implements get_children(),
_copy_internals(), compare() and _cache_key() for most Core elements.
Core elements with special needs like Select still implement
some of these methods directly however most of these methods
are no longer explicitly implemented.

The data-driven system is also applied to ORM elements that
take part in SQL expressions so that these objects, like mappers,
aliasedclass, query options, etc. can all participate in the
cache key process.

Still not considered is that this approach to defining traversibility
will be used to create some kind of generic introspection system
that works across Core / ORM.  It's also not clear if
real statement caching using the _cache_key() method is feasible,
if it is shown that running _cache_key() is nearly as expensive as
compiling in any case.    Because it is data driven, it is more
straightforward to optimize using inlined code, as is the case now,
as well as potentially using C code to speed it up.

In addition, the caching sytem now accommodates for anonymous
name labels, which is essential so that constructs which have
anonymous labels can be cacheable, that is, their position
within a statement in relation to other anonymous names causes
them to generate an integer counter relative to that construct
which will be the same every time.   Gathering of bound parameters
from any cache key generation is also now required as there is
no use case for a cache key that does not extract bound parameter
values.

Applies-to: #4639
Change-Id: I0660584def8627cad566719ee98d3be045db4b8d

1 files changed, 29 insertions, 7 deletions

diff --git a/lib/sqlalchemy/sql/base.py b/lib/sqlalchemy/sql/base.py
index 7e9199bfa..d11a3a313 100644
--- a/lib/sqlalchemy/sql/base.py
+++ b/lib/sqlalchemy/sql/base.py
@@ -14,6 +14,7 @@ import itertools
 import operator
 import re
 
+from .traversals import HasCacheKey  # noqa
 from .visitors import ClauseVisitor
 from .. import exc
 from .. import util
@@ -38,18 +39,41 @@ class Immutable(object):
     def _clone(self):
         return self
 
+    def _copy_internals(self, **kw):
+        pass
+
+
+class HasMemoized(object):
+    def _reset_memoizations(self):
+        self._memoized_property.expire_instance(self)
+
+    def _reset_exported(self):
+        self._memoized_property.expire_instance(self)
+
+    def _copy_internals(self, **kw):
+        super(HasMemoized, self)._copy_internals(**kw)
+        self._reset_memoizations()
+
 
 def _from_objects(*elements):
     return itertools.chain(*[element._from_objects for element in elements])
 
 
 def _generative(fn):
+    """non-caching _generative() decorator.
+
+    This is basically the legacy decorator that copies the object and
+    runs a method on the new copy.
+
+    """
+
     @util.decorator
-    def _generative(fn, *args, **kw):
+    def _generative(fn, self, *args, **kw):
         """Mark a method as generative."""
 
-        self = args[0]._generate()
-        fn(self, *args[1:], **kw)
+        self = self._generate()
+        x = fn(self, *args, **kw)
+        assert x is None, "generative methods must have no return value"
         return self
 
     decorated = _generative(fn)
@@ -357,10 +381,8 @@ class DialectKWArgs(object):
 
 
 class Generative(object):
-    """Allow a ClauseElement to generate itself via the
-    @_generative decorator.
-
-    """
+    """Provide a method-chaining pattern in conjunction with the
+    @_generative decorator."""
 
     def _generate(self):
         s = self.__class__.__new__(self.__class__)