dont pull filter_by from from_obj, entity is hopefully sufficient

Fixed 1.4 regression where :meth:`_orm.Query.filter_by` would not function
correctly on a :class:`_orm.Query` that was produced from
:meth:`_orm.Query.union`, :meth:`_orm.Query.from_self` or similar.

Fixes: #7239
Change-Id: I3a0c3fd71180b1bfb6bf855f436a29c729664082

1 files changed, 31 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/query.py b/lib/sqlalchemy/orm/query.py
index fcb9dae57..d48e34923 100644
--- a/lib/sqlalchemy/orm/query.py
+++ b/lib/sqlalchemy/orm/query.py
@@ -1707,12 +1707,42 @@ class Query(
             return None
 
     def _filter_by_zero(self):
+        """for the filter_by() method, return the target entity for which
+        we will attempt to derive an expression from based on string name.
+
+        """
         if self._legacy_setup_joins:
             _last_joined_entity = self._last_joined_entity
             if _last_joined_entity is not None:
                 return _last_joined_entity
 
-        if self._from_obj:
+        # discussion related to #7239
+        # special check determines if we should try to derive attributes
+        # for filter_by() from the "from object", i.e., if the user
+        # called query.select_from(some selectable).filter_by(some_attr=value).
+        # We don't want to do that in the case that methods like
+        # from_self(), select_entity_from(), or a set op like union() were
+        # called; while these methods also place a
+        # selectable in the _from_obj collection, they also set up
+        # the _set_base_alias boolean which turns on the whole "adapt the
+        # entity to this selectable" thing, meaning the query still continues
+        # to construct itself in terms of the lead entity that was passed
+        # to query(), e.g. query(User).from_self() is still in terms of User,
+        # and not the subquery that from_self() created.   This feature of
+        # "implicitly adapt all occurrences of entity X to some arbitrary
+        # subquery" is the main thing I am trying to do away with in 2.0 as
+        # users should now used aliased() for that, but I can't entirely get
+        # rid of it due to query.union() and other set ops relying upon it.
+        #
+        # compare this to the base Select()._filter_by_zero() which can
+        # just return self._from_obj[0] if present, because there is no
+        # "_set_base_alias" feature.
+        #
+        # IOW, this conditional essentially detects if
+        # "select_from(some_selectable)" has been called, as opposed to
+        # "select_entity_from()", "from_self()"
+        # or "union() / some_set_op()".
+        if self._from_obj and not self._compile_options._set_base_alias:
             return self._from_obj[0]
 
         return self._raw_columns[0]