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

2 files changed, 37 insertions, 1 deletions

diff --git a/test/sql/test_deprecations.py b/test/sql/test_deprecations.py
index 44135e373..9b74ab1fa 100644
--- a/test/sql/test_deprecations.py
+++ b/test/sql/test_deprecations.py
@@ -451,6 +451,17 @@ class SelectableTest(fixtures.TestBase, AssertsCompiledSQL):
             "deprecated"
         )
 
+    def test_froms_renamed(self):
+        t1 = table("t1", column("q"))
+
+        stmt = select(t1)
+
+        with testing.expect_deprecated(
+            r"The Select.froms attribute is moved to the "
+            r"Select.get_final_froms\(\) method."
+        ):
+            eq_(stmt.froms, [t1])
+
     def test_select_list_argument(self):
 
         with testing.expect_deprecated_20(
diff --git a/test/sql/test_selectable.py b/test/sql/test_selectable.py
index 5a94d4038..b76873490 100644
--- a/test/sql/test_selectable.py
+++ b/test/sql/test_selectable.py
@@ -1,5 +1,4 @@
 """Test various algorithmic properties of selectables."""
-
 from sqlalchemy import and_
 from sqlalchemy import bindparam
 from sqlalchemy import Boolean
@@ -590,6 +589,32 @@ class SelectableTest(
             "table1.col3, table1.colx FROM table1) AS anon_1",
         )
 
+    @testing.combinations(
+        (
+            [table1.c.col1],
+            [table1.join(table2)],
+            [table1.join(table2)],
+            [table1],
+        ),
+        ([table1], [table2], [table1, table2], [table1]),
+        (
+            [table1.c.col1, table2.c.col1],
+            [],
+            [table1, table2],
+            [table1, table2],
+        ),
+    )
+    def test_froms_accessors(
+        self, cols_expr, select_from, exp_final_froms, exp_cc_froms
+    ):
+        """tests for #6808"""
+        s1 = select(*cols_expr).select_from(*select_from)
+
+        for ff, efp in util.zip_longest(s1.get_final_froms(), exp_final_froms):
+            assert ff.compare(efp)
+
+        eq_(s1.columns_clause_froms, exp_cc_froms)
+
     def test_scalar_subquery_from_subq_same_source(self):
         s1 = select(table1.c.col1)