Added Abstract Base Classes

Thanks to @theojulienne for the code :) #108

mongoengine/base.py

@@ -253,7 +253,16 @@ class TopLevelDocumentMetaclass(DocumentMetaclass):
         # __metaclass__ is only set on the class with the __metaclass__
         # attribute (i.e. it is not set on subclasses). This differentiates
         # 'real' documents from the 'Document' class
-        if attrs.get('__metaclass__') == TopLevelDocumentMetaclass:
+        #
+        # Also assume a class is abstract if it has abstract set to True in
+        # its meta dictionary. This allows custom Document superclasses.
+        if (attrs.get('__metaclass__') == TopLevelDocumentMetaclass or
+            ('meta' in attrs and attrs['meta'].get('abstract', False))):
+            # Make sure no base class was non-abstract
+            non_abstract_bases = [b for b in bases
+                if hasattr(b,'_meta') and not b._meta.get('abstract', False)]
+            if non_abstract_bases:
+                raise ValueError("Abstract document cannot have non-abstract base")
             return super_new(cls, name, bases, attrs)
 
         collection = name.lower()
@@ -276,6 +285,7 @@ class TopLevelDocumentMetaclass(DocumentMetaclass):
                 base_indexes += base._meta.get('indexes', [])
 
         meta = {
+            'abstract': False,
             'collection': collection,
             'max_documents': None,
             'max_size': None,

tests/document.py

@@ -29,6 +29,9 @@ class DocumentTest(unittest.TestCase):
             age = IntField()
         self.Person = Person
 
+    def tearDown(self):
+        self.Person.drop_collection()
+
     def test_drop_collection(self):
         """Ensure that the collection may be dropped from the database.
         """
@@ -188,6 +191,34 @@ class DocumentTest(unittest.TestCase):
         self.assertFalse('_cls' in comment.to_mongo())
         self.assertFalse('_types' in comment.to_mongo())
 
+    def test_abstract_documents(self):
+        """Ensure that a document superclass can be marked as abstract
+        thereby not using it as the name for the collection."""
+
+        class Animal(Document):
+            name = StringField()
+            meta = {'abstract': True}
+
+        class Fish(Animal): pass
+        class Guppy(Fish): pass
+
+        class Mammal(Animal):
+            meta = {'abstract': True}
+        class Human(Mammal): pass
+
+        self.assertFalse('collection' in Animal._meta)
+        self.assertFalse('collection' in Mammal._meta)
+
+        self.assertEqual(Fish._meta['collection'], 'fish')
+        self.assertEqual(Guppy._meta['collection'], 'fish')
+        self.assertEqual(Human._meta['collection'], 'human')
+
+        def create_bad_abstract():
+            class EvilHuman(Human):
+                evil = BooleanField(default=True)
+                meta = {'abstract': True}
+        self.assertRaises(ValueError, create_bad_abstract)
+
     def test_collection_name(self):
         """Ensure that a collection with a specified name may be used.
         """
@@ -907,9 +938,6 @@ class DocumentTest(unittest.TestCase):
         A.drop_collection()
         B.drop_collection()
 
-    def tearDown(self):
-        self.Person.drop_collection()
-
     def test_document_hash(self):
         """Test document in list, dict, set
         """