Add a third labeling mode for SELECT statements

Enhanced the disambiguating labels feature of the
:func:`~.sql.expression.select` construct such that when a select statement
is used in a subquery, repeated column names from different tables are now
automatically labeled with a unique label name, without the need to use the
full "apply_labels()" feature that conbines tablename plus column name.
The disambigated labels are available as plain string keys in the .c
collection of the subquery, and most importantly the feature allows an ORM
:func:`.orm.aliased` construct against the combination of an entity and an
arbitrary subquery to work correctly, targeting the correct columns despite
same-named columns in the source tables, without the need for an "apply
labels" warning.

The existing labeling style is now called
LABEL_STYLE_TABLENAME_PLUS_COL.  This labeling style will remain used
throughout the ORM as has been the case for over a decade, however,
the new disambiguation scheme could theoretically replace this scheme
entirely.  The new scheme would dramatically alter how SQL looks
when rendered from the ORM to be more succinct but arguably harder
to read.

The tablename_columnname scheme used by Join.c is unaffected here,
as that's still hardcoded to that scheme.

Fixes: #5221
Change-Id: Ib47d9e0f35046b3afc77bef6e65709b93d0c3026

1 files changed, 21 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/base.py b/lib/sqlalchemy/sql/base.py
index f093cad90..f39f932e9 100644
--- a/lib/sqlalchemy/sql/base.py
+++ b/lib/sqlalchemy/sql/base.py
@@ -638,6 +638,27 @@ class Executable(Generative):
             return None
 
 
+class prefix_anon_map(dict):
+    """A map that creates new keys for missing key access.
+
+    Considers keys of the form "<ident> <name>" to produce
+    new symbols "<name>_<index>", where "index" is an incrementing integer
+    corresponding to <name>.
+
+    Inlines the approach taken by :class:`sqlalchemy.util.PopulateDict` which
+    is otherwise usually used for this type of operation.
+
+    """
+
+    def __missing__(self, key):
+        (ident, derived) = key.split(" ", 1)
+        anonymous_counter = self.get(derived, 1)
+        self[derived] = anonymous_counter + 1
+        value = derived + "_" + str(anonymous_counter)
+        self[key] = value
+        return value
+
+
 class SchemaEventTarget(object):
     """Base class for elements that are the targets of :class:`.DDLEvents`
     events.