Add "existing" populators for subqueryload

Fixed bug in subquery loading where an object encountered as an
"existing" row, e.g. already loaded from a different path in the
same query, would not invoke subquery loaders for unloaded attributes
that specified this loading.  This issue is in the same area
as that of :ticket:`3431`, :ticket:`3811` which involved
similar issues with joined loading.

Change-Id: If111a76b0812010905b0ac811276a816779d297f
Fixes: #3854

1 files changed, 17 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/strategies.py b/lib/sqlalchemy/orm/strategies.py
index ace82871f..4e3574b54 100644
--- a/lib/sqlalchemy/orm/strategies.py
+++ b/lib/sqlalchemy/orm/strategies.py
@@ -1087,7 +1087,15 @@ class SubqueryLoader(AbstractRelationshipLoader):
             state.get_impl(self.key).\
                 set_committed_value(state, dict_, collection)
 
-        populators["new"].append((self.key, load_collection_from_subq))
+        def load_collection_from_subq_existing_row(state, dict_, row):
+            if self.key not in dict_:
+                load_collection_from_subq(state, dict_, row)
+
+        populators["new"].append(
+            (self.key, load_collection_from_subq))
+        populators["existing"].append(
+            (self.key, load_collection_from_subq_existing_row))
+
         if context.invoke_all_eagers:
             populators["eager"].append((self.key, collections.loader))
 
@@ -1108,7 +1116,14 @@ class SubqueryLoader(AbstractRelationshipLoader):
             state.get_impl(self.key).\
                 set_committed_value(state, dict_, scalar)
 
-        populators["new"].append((self.key, load_scalar_from_subq))
+        def load_scalar_from_subq_existing_row(state, dict_, row):
+            if self.key not in dict_:
+                load_scalar_from_subq(state, dict_, row)
+
+        populators["new"].append(
+            (self.key, load_scalar_from_subq))
+        populators["existing"].append(
+            (self.key, load_scalar_from_subq_existing_row))
         if context.invoke_all_eagers:
             populators["eager"].append((self.key, collections.loader))