Add devref for Relocating DB models

This patch will add devref for relocating db models and
it also explains why that change is required. It will also
add a directory named models under tree neutron/db/.

Change-Id: I160cecb76fb736307e5937122c84cdc0546207b6
Related-Bug: #1597913

doc/source/devref/db_models.rst

@@ -0,0 +1,49 @@
+..
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+
+Relocation of Database Models
+=============================
+
+This document is intended to track and notify developers that db models in
+neutron will be centralized and moved to a new tree under neutron/db/models.
+This was discussed in [1]. The reason for relocating db models is to solve
+the cyclic import issue while implementing oslo versioned objects for
+resources in neutron.
+
+The reason behind this relocation is Mixin class and db models for some
+resources in neutron are in same module. In Mixin classes, there are methods
+which provide functionality of fetching, adding, updating and deleting data
+via queries. These queries will be replaced with use of versioned objects and
+definition of versioned object will be using db models. So object files will
+be importing models and Mixin need to import those objects which will end up
+in cyclic import.
+
+Structure of Model Definitions
+------------------------------
+
+We have decided to move all models definitions to neutron/db/models/
+with no futher nesting after that point. The deprecation method to move
+models is already been added to avoid breakage of third party plugins using
+those models. All relocated models need to use deprecate method that
+will generate a warning and return new class for use of old class. Some
+examples of relocated models [2] and [3]. In future if you define new models
+please make sure they are separated from mixins and are under tree
+neutron/db/models/ .
+
+References
+~~~~~~~~~~
+
+[1]. https://www.mail-archive.com/openstack-dev@lists.openstack.org/msg88910.html
+[2]. https://review.openstack.org/#/c/348562/
+[3]. https://review.openstack.org/#/c/348757/

neutron/db/models/README

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+This directory is designed to contain all SQLAlchemy models shipped with core
+Neutron. The expected directory structure is flat, meaning there should be no
+subdirectories. Module names should use singular forms for nouns (port.py, not
+ports.py).