Add new "expanding" feature to bindparam()

Added a new kind of :func:`.bindparam` called "expanding".  This is
for use in ``IN`` expressions where the list of elements is rendered
into individual bound parameters at statement execution time, rather
than at statement compilation time.  This allows both a single bound
parameter name to be linked to an IN expression of multiple elements,
as well as allows query caching to be used with IN expressions.  The
new feature allows the related features of "select in" loading and
"polymorphic in" loading to make use of the baked query extension
to reduce call overhead.   This feature should be considered to be
**experimental** for 1.2.

Fixes: #3953
Change-Id: Ie708414a3ab9c0af29998a2c7f239ff7633b1f6e

1 files changed, 28 insertions, 6 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index cc4248009..6da064797 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -350,6 +350,14 @@ class SQLCompiler(Compiled):
     columns with the table name (i.e. MySQL only)
     """
 
+    contains_expanding_parameters = False
+    """True if we've encountered bindparam(..., expanding=True).
+
+    These need to be converted before execution time against the
+    string statement.
+
+    """
+
     ansi_bind_rules = False
     """SQL 92 doesn't allow bind parameters to be used
     in the columns clause of a SELECT, nor does it allow
@@ -370,8 +378,14 @@ class SQLCompiler(Compiled):
     True unless using an unordered TextAsFrom.
     """
 
-    insert_prefetch = update_prefetch = ()
+    _numeric_binds = False
+    """
+    True if paramstyle is "numeric".  This paramstyle is trickier than
+    all the others.
 
+    """
+
+    insert_prefetch = update_prefetch = ()
 
     def __init__(self, dialect, statement, column_keys=None,
                  inline=False, **kwargs):
@@ -418,6 +432,7 @@ class SQLCompiler(Compiled):
         self.positional = dialect.positional
         if self.positional:
             self.positiontup = []
+            self._numeric_binds = dialect.paramstyle == "numeric"
         self.bindtemplate = BIND_TEMPLATES[dialect.paramstyle]
 
         self.ctes = None
@@ -439,7 +454,7 @@ class SQLCompiler(Compiled):
         ) and statement._returning:
             self.returning = statement._returning
 
-        if self.positional and dialect.paramstyle == 'numeric':
+        if self.positional and self._numeric_binds:
             self._apply_numbered_params()
 
     @property
@@ -492,7 +507,8 @@ class SQLCompiler(Compiled):
         return dict(
             (key, value) for key, value in
             ((self.bind_names[bindparam],
-              bindparam.type._cached_bind_processor(self.dialect))
+              bindparam.type._cached_bind_processor(self.dialect)
+              )
              for bindparam in self.bind_names)
             if value is not None
         )
@@ -1238,7 +1254,8 @@ class SQLCompiler(Compiled):
 
         self.binds[bindparam.key] = self.binds[name] = bindparam
 
-        return self.bindparam_string(name, **kwargs)
+        return self.bindparam_string(
+            name, expanding=bindparam.expanding, **kwargs)
 
     def render_literal_bindparam(self, bindparam, **kw):
         value = bindparam.effective_value
@@ -1300,13 +1317,18 @@ class SQLCompiler(Compiled):
         self.anon_map[derived] = anonymous_counter + 1
         return derived + "_" + str(anonymous_counter)
 
-    def bindparam_string(self, name, positional_names=None, **kw):
+    def bindparam_string(
+            self, name, positional_names=None, expanding=False, **kw):
         if self.positional:
             if positional_names is not None:
                 positional_names.append(name)
             else:
                 self.positiontup.append(name)
-        return self.bindtemplate % {'name': name}
+        if expanding:
+            self.contains_expanding_parameters = True
+            return "([EXPANDING_%s])" % name
+        else:
+            return self.bindtemplate % {'name': name}
 
     def visit_cte(self, cte, asfrom=False, ashint=False,
                   fromhints=None,