impl prefetched

peewee.py

@@ -5175,6 +5175,18 @@ def save(self, force_insert=False, only=None):
     def is_dirty(self):
         return bool(self._dirty)
 
+    def prefetched(self, fk):
+        '''
+            Accepts a ForeignKeyField.
+            Returns true/false representing if accessing this attribute will trigger a database query. 
+        '''
+        if isinstance(self, fk.model_class):
+            if getattr(self, fk.db_column) is None: return True
+            return fk.name in self._obj_cache
+        if isinstance(self, fk.rel_model):
+            return isinstance(getattr(self, fk._get_related_name()), list)
+        raise AttributeError('ForeignKeyField %s is not related to this model.' % fk)
+
     @property
     def dirty_fields(self):
         return [f for f in self._meta.sorted_fields if f.name in self._dirty]

playhouse/tests/test_query_results.py

@@ -484,6 +484,51 @@ def test_aliasing_values(self):
         self.assertEqual(results, [('u1', 'u1'), ('u2', 'u2')])
 
 
+class TestPrefetchDetection(ModelTestCase):
+    requires = [Parent, Child, Orphan]
+
+    def setUp(self):
+        super(TestPrefetchDetection, self).setUp()
+        u1 = Parent.create(data='u1')
+        u2 = Child.create(parent=u1, data='u2')
+        u3 = Orphan.create(data='u3')
+
+    def test_prefetched(self):
+        u2 = (Child
+             .select(Child, Parent)
+             .join(Parent)
+             .first())
+        self.assertTrue(u2.prefetched(Child.parent))
+
+    def test_not_prefetched(self):
+        u2 = Child.select().first()
+        self.assertFalse(u2.prefetched(Child.parent))
+
+    def test_prefetched_null(self):
+        u3 = (Orphan
+             .select(Orphan, Parent)
+             .join(Parent, JOIN.LEFT_OUTER)
+             .first())
+        self.assertTrue(u3.prefetched(Orphan.parent))
+
+    def test_not_prefetched_null(self):
+        u3 = Orphan.select().first()
+        # even though it's not joined, we know the model
+        # is null because the FK value is null.
+        self.assertTrue(u3.prefetched(Orphan.parent))
+
+    def test_prefetched_nonsense(self):
+        u2 = Child.select().first()
+        def f():
+          u2.prefetched(Orphan.parent)
+        self.assertRaises(AttributeError, f)
+
+    def test_prefetched_set(self):
+        u2 = Parent.ALL.plus(Child.parent).first()
+        self.assertTrue(u2.prefetched(Child.parent))
+
+
+
 class TestJoinedInstanceConstruction(ModelTestCase):
     requires = [Blog, User, Relationship]