chain joins from SelectState context, not Select

Fixed issue in new :meth:`_sql.Select.join` method where chaining from the
current JOIN wasn't looking at the right state, causing an expression like
"FROM a JOIN b <onclause>, b JOIN c <onclause>" rather than
"FROM a JOIN b <onclause> JOIN c <onclause>".

Added :meth:`_sql.Select.outerjoin_from` method to complement
:meth:`_sql.Select.join_from`.

Fixes: #5858
Change-Id: I1346ebe0963bbd1e4bf868650e3ee1d6d3072f04

1 files changed, 14 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/selectable.py b/lib/sqlalchemy/sql/selectable.py
index b4f6c8bc9..8e478583f 100644
--- a/lib/sqlalchemy/sql/selectable.py
+++ b/lib/sqlalchemy/sql/selectable.py
@@ -3944,7 +3944,7 @@ class SelectState(util.MemoizedSlots, CompileState):
 
         replace_from_obj_index = None
 
-        from_clauses = self.statement._from_obj
+        from_clauses = self.from_clauses
 
         if from_clauses:
 
@@ -4644,6 +4644,19 @@ class Select(
             (target, onclause, None, {"isouter": isouter, "full": full}),
         )
 
+    def outerjoin_from(self, from_, target, onclause=None, full=False):
+        r"""Create a SQL LEFT OUTER JOIN against this :class:`_expression.Select`
+        object's criterion
+        and apply generatively, returning the newly resulting
+        :class:`_expression.Select`.
+
+        Usage is the same as that of :meth:`_selectable.Select.join_from`.
+
+        """
+        return self.join_from(
+            from_, target, onclause=onclause, isouter=True, full=full
+        )
+
     @_generative
     def join_from(
         self, from_, target, onclause=None, isouter=False, full=False