Call property mapper to materialize lazy attrs

A Relationship property from a mapper may not have all expected
attributes available immediately. Calling the `prop.mapper` property
will cause these missing attributes to materialize.

flask_admin/contrib/sqla/form.py

@@ -670,6 +670,13 @@ def _calculate_mapping_key_pair(self, model, info):
         reverse_prop = None
 
         for prop in target_mapper.iterate_properties:
+            try:
+                # Loading the mapper property will populate a lazy direction
+                # attribute
+                prop.mapper
+            except AttributeError:
+                continue
+
             if hasattr(prop, 'direction') and prop.direction.name in ('MANYTOONE', 'MANYTOMANY'):
                 if issubclass(model, prop.mapper.class_):
                     reverse_prop = prop