models: Make sure parent fields are used by children.

Without this change any sub-class would end up with an empty _fields and none
of the actual fields would be writable even internally.

mopidy/models/immutable.py

@@ -15,7 +15,9 @@ class ImmutableObjectMeta(type):
 
     def __new__(cls, name, bases, attrs):
         fields = {}
-        for key, value in attrs.items():
+        for base in bases:  # Copy parent fields over to our state
+            fields.update(getattr(base, '_fields', {}))
+        for key, value in attrs.items():  # Add our own fields
             if isinstance(value, Field):
                 fields[key] = '_' + key
                 value._name = key

tests/models/test_models.py

@@ -19,6 +19,7 @@ class InheritanceTest(unittest.TestCase):
             pass
 
     def test_sub_class_can_have_its_own_slots(self):
+        # Needed for things like SpotifyTrack in mopidy-spotify 1.x
 
         class Foo(Track):
             __slots__ = ('_foo',)
@@ -26,6 +27,17 @@ class InheritanceTest(unittest.TestCase):
         f = Foo()
         f._foo = 123
 
+    def test_sub_class_can_be_initialized(self):
+        # Fails with following error if fields are not handled across classes.
+        #   TypeError: __init__() got an unexpected keyword argument "type"
+        # Essentially this is testing that sub-classes take parent _fields into
+        # account.
+
+        class Foo(Ref):
+            pass
+
+        Foo.directory()
+
 
 class CachingTest(unittest.TestCase):