Ensure queries and scans return model subclasses when using discrimin…

…ators. (#873)

docs/release_notes.rst

@@ -1,7 +1,7 @@
 Release Notes
 =============
 
-v5.0.0b2
+v5.0.0b3
 -------------------
 
 :date: 2020-xx-xx

pynamodb/__init__.py

@@ -7,4 +7,4 @@
 """
 __author__ = 'Jharrod LaFon'
 __license__ = 'MIT'
-__version__ = '5.0.0b2'
+__version__ = '5.0.0b3'

pynamodb/attributes.py

@@ -415,6 +415,9 @@ def register_class(self, cls: type, discriminator: Any):
 
         self._discriminator_map[discriminator] = cls
 
+    def get_registered_subclasses(self, cls: type) -> List[type]:
+        return [k for k in self._class_map.keys() if issubclass(k, cls)]
+
     def get_discriminator(self, cls: type) -> Optional[Any]:
         return self._class_map.get(cls)
 

pynamodb/models.py

@@ -575,10 +575,10 @@ def count(
         else:
             hash_key = cls._serialize_keys(hash_key)[0]
 
-        # If this class has a discriminator value, filter the query to only return instances of this class.
+        # If this class has a discriminator attribute, filter the query to only return instances of this class.
         discriminator_attr = cls._get_discriminator_attribute()
-        if discriminator_attr and discriminator_attr.get_discriminator(cls):
-            filter_condition &= discriminator_attr == cls
+        if discriminator_attr:
+            filter_condition &= discriminator_attr.is_in(*discriminator_attr.get_registered_subclasses(cls))
 
         query_args = (hash_key,)
         query_kwargs = dict(
@@ -640,10 +640,10 @@ def query(
         else:
             hash_key = cls._serialize_keys(hash_key)[0]
 
-        # If this class has a discriminator value, filter the query to only return instances of this class.
+        # If this class has a discriminator attribute, filter the query to only return instances of this class.
         discriminator_attr = cls._get_discriminator_attribute()
-        if discriminator_attr and discriminator_attr.get_discriminator(cls):
-            filter_condition &= discriminator_attr == cls
+        if discriminator_attr:
+            filter_condition &= discriminator_attr.is_in(*discriminator_attr.get_registered_subclasses(cls))
 
         if page_size is None:
             page_size = limit
@@ -697,10 +697,10 @@ def scan(
         :param rate_limit: If set then consumed capacity will be limited to this amount per second
         :param attributes_to_get: If set, specifies the properties to include in the projection expression
         """
-        # If this class has a discriminator value, filter the scan to only return instances of this class.
+        # If this class has a discriminator attribute, filter the scan to only return instances of this class.
         discriminator_attr = cls._get_discriminator_attribute()
-        if discriminator_attr and discriminator_attr.get_discriminator(cls):
-            filter_condition &= discriminator_attr == cls
+        if discriminator_attr:
+            filter_condition &= discriminator_attr.is_in(*discriminator_attr.get_registered_subclasses(cls))
 
         if page_size is None:
             page_size = limit

tests/test_model.py

@@ -1553,6 +1553,10 @@ class Meta:
         class ChildModel(ParentModel, discriminator='Child'):
             foo = UnicodeAttribute()
 
+        # register a model that subclasses Child to ensure queries return model subclasses
+        class GrandchildModel(ChildModel, discriminator='Grandchild'):
+            bar = UnicodeAttribute()
+
         with patch(PATCH_METHOD) as req:
             req.return_value = {
                 "Table": {
@@ -1588,7 +1592,7 @@ class ChildModel(ParentModel, discriminator='Child'):
                 pass
             params = {
                 'KeyConditionExpression': '#0 = :0',
-                'FilterExpression': '#1 = :1',
+                'FilterExpression': '#1 IN (:1, :2)',
                 'ExpressionAttributeNames': {
                     '#0': 'id',
                     '#1': 'cls'
@@ -1599,6 +1603,9 @@ class ChildModel(ParentModel, discriminator='Child'):
                     },
                     ':1': {
                         'S': u'Child'
+                    },
+                    ':2': {
+                        'S': u'Grandchild'
                     }
                 },
                 'ReturnConsumedCapacity': 'TOTAL',