Added MapField, similar to DictField

Similar to DictField except the value of each entry is always of a certain
(declared) field type.

Thanks again to @theojulienne for the code #108

tests/fields.py

@@ -825,5 +825,66 @@ class FieldTest(unittest.TestCase):
         self.assertEqual(d2.data, {})
         self.assertEqual(d2.data2, {})
 
+    def test_mapfield(self):
+        """Ensure that the MapField handles the declared type."""
+
+        class Simple(Document):
+            mapping = MapField(IntField())
+
+        Simple.drop_collection()
+
+        e = Simple()
+        e.mapping['someint'] = 1
+        e.save()
+
+        def create_invalid_mapping():
+            e.mapping['somestring'] = "abc"
+            e.save()
+
+        self.assertRaises(ValidationError, create_invalid_mapping)
+
+        def create_invalid_class():
+            class NoDeclaredType(Document):
+                mapping = MapField()
+
+        self.assertRaises(ValidationError, create_invalid_class)
+
+        Simple.drop_collection()
+
+    def test_complex_mapfield(self):
+        """Ensure that the MapField can handle complex declared types."""
+
+        class SettingBase(EmbeddedDocument):
+            pass
+
+        class StringSetting(SettingBase):
+            value = StringField()
+
+        class IntegerSetting(SettingBase):
+            value = IntField()
+
+        class Extensible(Document):
+            mapping = MapField(EmbeddedDocumentField(SettingBase))
+
+        Extensible.drop_collection()
+
+        e = Extensible()
+        e.mapping['somestring'] = StringSetting(value='foo')
+        e.mapping['someint'] = IntegerSetting(value=42)
+        e.save()
+
+        e2 = Extensible.objects.get(id=e.id)
+        self.assertTrue(isinstance(e2.mapping['somestring'], StringSetting))
+        self.assertTrue(isinstance(e2.mapping['someint'], IntegerSetting))
+
+        def create_invalid_mapping():
+            e.mapping['someint'] = 123
+            e.save()
+
+        self.assertRaises(ValidationError, create_invalid_mapping)
+
+        Extensible.drop_collection()
+
+
 if __name__ == '__main__':
     unittest.main()