Fixing a potential FK error when doing bulk updates (#606)

I hit this upgrade issue in production where the FK for user wasn't
allowing null. Perhaps it is specific to our environment but I'd rather
fix this.

caravel/migrations/versions/d8bc074f7aad_add_new_field_is_restricted_to_.py

@@ -23,12 +23,11 @@ def upgrade():
     bind = op.get_bind()
     session = db.Session(bind=bind)
 
-    session.query(models.DruidMetric).update({
-        'is_restricted': False
-    })
-    session.query(models.SqlMetric).update({
-        'is_restricted': False
-    })
+    for obj in session.query(models.DruidMetric).all():
+        obj.is_restricted = False
+
+    for obj in session.query(models.SqlMetric).all():
+        obj.is_restricted = False
 
     session.commit()
     session.close()