add columns_clause_froms and related use cases

Added new attribute :attr:`_sql.Select.columns_clause_froms` that will
retrieve the FROM list implied by the columns clause of the
:class:`_sql.Select` statement. This differs from the old
:attr:`_sql.Select.froms` collection in that it does not perform any ORM
compilation steps, which necessarily deannotate the FROM elements and do
things like compute joinedloads etc., which makes it not an appropriate
candidate for the :meth:`_sql.Select.select_from` method. Additionally adds
a new parameter
:paramref:`_sql.Select.with_only_columns.maintain_column_froms` that
transfers this collection to :meth:`_sql.Select.select_from` before
replacing the columns collection.

In addition, the :attr:`_sql.Select.froms` is renamed to
:meth:`_sql.Select.get_final_froms`, to stress that this collection is not
a simple accessor and is instead calculated given the full state of the
object, which can be an expensive call when used in an ORM context.

Additionally fixes a regression involving the
:func:`_orm.with_only_columns` function to support applying criteria to
column elements that were replaced with either
:meth:`_sql.Select.with_only_columns` or :meth:`_orm.Query.with_entities` ,
which had broken as part of :ticket:`6503` released in 1.4.19.

Fixes: #6808

Change-Id: Ib5d66cce488bbaca06dab4f68fb5cdaa73e8823e

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/context.py b/lib/sqlalchemy/orm/context.py
index c4b695687..603477819 100644
--- a/lib/sqlalchemy/orm/context.py
+++ b/lib/sqlalchemy/orm/context.py
@@ -4,6 +4,8 @@
 #
 # This module is part of SQLAlchemy and is released under
 # the MIT License: https://www.opensource.org/licenses/mit-license.php
+import itertools
+
 from . import attributes
 from . import interfaces
 from . import loading
@@ -873,6 +875,19 @@ class ORMSelectCompileState(ORMCompileState, SelectState):
                     yield elem
 
     @classmethod
+    def get_columns_clause_froms(cls, statement):
+        return cls._normalize_froms(
+            itertools.chain.from_iterable(
+                element._from_objects
+                if "parententity" not in element._annotations
+                else [
+                    element._annotations["parententity"].__clause_element__()
+                ]
+                for element in statement._raw_columns
+            )
+        )
+
+    @classmethod
     @util.preload_module("sqlalchemy.orm.query")
     def from_statement(cls, statement, from_statement):
         query = util.preloaded.orm_query