[1.0.X] Made it explicit if you accidentally override a Field from a parent model.

This was always not working reliably (model initialization and serialization
were two of the problems). Now, it's an explicit error. Also, documented.

Fixed #10252.

Backport of r9974 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9975 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/base.py

@@ -94,6 +94,16 @@ class ModelBase(type):
                          new_class._meta.virtual_fields
             field_names = set([f.name for f in new_fields])
 
+            parent_fields = base._meta.local_fields + base._meta.local_many_to_many
+            # Check for clashes between locally declared fields and those
+            # on the base classes (we cannot handle shadowed fields at the
+            # moment).
+            for field in parent_fields:
+                if field.name in field_names:
+                    raise FieldError('Local field %r in class %r clashes '
+                                     'with field of similar name from '
+                                     'base class %r' %
+                                        (field.name, name, base.__name__))
             if not base._meta.abstract:
                 # Concrete classes...
                 if base in o2o_map:
@@ -107,16 +117,7 @@ class ModelBase(type):
 
             else:
                 # .. and abstract ones.
-
-                # Check for clashes between locally declared fields and those
-                # on the ABC.
-                parent_fields = base._meta.local_fields + base._meta.local_many_to_many
                 for field in parent_fields:
-                    if field.name in field_names:
-                        raise FieldError('Local field %r in class %r clashes '\
-                                         'with field of similar name from '\
-                                         'abstract base class %r' % \
-                                            (field.name, name, base.__name__))
                     new_class.add_to_class(field.name, copy.deepcopy(field))
 
                 # Pass any non-abstract parent classes onto child.
@@ -131,7 +132,8 @@ class ModelBase(type):
                     new_manager = manager._copy_to_model(new_class)
                     new_class.add_to_class(mgr_name, new_manager)
 
-            # Inherit virtual fields (like GenericForeignKey) from the parent class
+            # Inherit virtual fields (like GenericForeignKey) from the parent
+            # class
             for field in base._meta.virtual_fields:
                 if base._meta.abstract and field.name in field_names:
                     raise FieldError('Local field %r in class %r clashes '\

docs/topics/db/models.txt

@@ -1006,3 +1006,32 @@ field or method to every class that inherits the mix-in. Try to keep your
 inheritance hierarchies as simple and straightforward as possible so that you
 won't have to struggle to work out where a particular piece of information is
 coming from.
+
+Field name "hiding" is not permitted
+-------------------------------------
+
+In normal Python class inheritance, it is permissible for a child class to
+override any attribute from the parent class. In Django, this is not permitted
+for attributes that are :class:`~django.db.models.fields.Field` instances (at
+least, not at the moment). If a base class has a field called ``author``, you
+cannot create another model field called ``author`` in any class that inherits
+from that base class.
+
+Overriding fields in a parent model leads to difficulties in areas such as
+initialising new instances (specifying which field is being intialised in
+``Model.__init__``) and serialization. These are features which normal Python
+class inheritance doesn't have to deal with in quite the same way, so the
+difference between Django model inheritance and Python class inheritance isn't
+merely arbitrary.
+
+This restriction only applies to attributes which are
+:class:`~django.db.models.fields.Field` instances. Normal Python attributes
+can be overridden if you wish. It also only applies to the name of the
+attribute as Python sees it: if you are manually specifying the database
+column name, you can have the same column name appearing in both a child and
+an ancestor model for multi-table inheritance (they are columns in two
+different database tables).
+
+Django will raise a ``FieldError`` exception if you override any model field
+in any ancestor model.
+