Optimize DB accesses by using an SQL JOIN when retrieving a user.

`Datastore.find_user()` is used on every page load to verify the user in
the current session; until now, `find_user()` would trigger 2+ DB
accesses, since the `roles` relationship uses lazy loading by default.
This is useful to reduce the rows accessed when retrieving multiple
users from the DB.

However, in `find_user()` we always retrieve only one, so
it makes sense to retrieve all roles in one SQL operation instead of
always triggering 2 or more.

This would be unnecessary if the `roles` relationship was defined with
`lazy="joined"`. However, that would greatly increase the number of rows
accessed on all queries (even when querying all users), so I think it's
better to be selective and optimize only these accesses which happen on
every page load.

flask_security/datastore.py

@@ -234,12 +234,18 @@ def __init__(self, db, user_model, role_model):
 
     def get_user(self, identifier):
         from sqlalchemy import func as alchemyFn
+        from sqlalchemy.orm import joinedload
         if self._is_numeric(identifier):
-            return self.user_model.query.get(identifier)
+            return self.user_model.query\
+                                  .options(joinedload('roles'))\
+                                  .get(identifier)
         for attr in get_identity_attributes():
             query = alchemyFn.lower(getattr(self.user_model, attr)) \
                 == alchemyFn.lower(identifier)
-            rv = self.user_model.query.filter(query).first()
+            rv = self.user_model.query\
+                                .options(joinedload('roles'))\
+                                .filter(query)\
+                                .first()
             if rv is not None:
                 return rv
 
@@ -251,7 +257,11 @@ def _is_numeric(self, value):
         return True
 
     def find_user(self, **kwargs):
-        return self.user_model.query.filter_by(**kwargs).first()
+        from sqlalchemy.orm import joinedload
+        return self.user_model.query\
+                              .options(joinedload('roles'))\
+                              .filter_by(**kwargs)\
+                              .first()
 
     def find_role(self, role):
         return self.role_model.query.filter_by(name=role).first()