remove should_nest behavior for contains_eager()

Fixed regression for 1.4 in :func:`_orm.contains_eager` where the "wrap in
subquery" logic of :func:`_orm.joinedload` would be inadvertently triggered
for use of the :func:`_orm.contains_eager` function with similar statements
(e.g. those that use ``distinct()``, ``limit()`` or ``offset()``). This is
not appropriate for :func:`_orm.contains_eager` which has always had the
contract that the user-defined SQL statement is unmodified with the
exception of adding the appropriate columns.

Also includes an adjustment to the assertion in Label._make_proxy()
which was there to prevent a fixed label name from being anonymized;
if the label is already anonymous, the change should proceed.
This logic was being hit before the contains_eager behavior was
adjusted. With the adjustment, this code is not used.

Fixes: #8569
Change-Id: I161e65041c0162fd2b83cbef40f57a50fcfaf0fd
(cherry picked from commit 57b400f07951f0ae8651ca38338ec5be1d222c7e)

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index 944c114a6..a014b2f41 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -1965,6 +1965,9 @@ class JoinedLoader(AbstractRelationshipLoader):
         )
 
         if user_defined_adapter is not False:
+
+            # setup an adapter but dont create any JOIN, assume it's already
+            # in the query
             (
                 clauses,
                 adapter,
@@ -1976,6 +1979,11 @@ class JoinedLoader(AbstractRelationshipLoader):
                 adapter,
                 user_defined_adapter,
             )
+
+            # don't do "wrap" for multi-row, we want to wrap
+            # limited/distinct SELECT,
+            # because we want to put the JOIN on the outside.
+
         else:
             # if not via query option, check for
             # a cycle
@@ -1986,6 +1994,7 @@ class JoinedLoader(AbstractRelationshipLoader):
                 elif path.contains_mapper(self.mapper):
                     return
 
+            # add the JOIN and create an adapter
             (
                 clauses,
                 adapter,
@@ -2002,6 +2011,10 @@ class JoinedLoader(AbstractRelationshipLoader):
                 chained_from_outerjoin,
             )
 
+            # for multi-row, we want to wrap limited/distinct SELECT,
+            # because we want to put the JOIN on the outside.
+            compile_state.eager_adding_joins = True
+
         with_poly_entity = path.get(
             compile_state.attributes, "path_with_polymorphic", None
         )