[Django] #28438: Initial migration creates fields not listed in the migration if mixin class changes

[Django]

#28438: Initial migration creates fields not listed in the migration if mixin 
class
changes
-----------------------------------------+------------------------
               Reporter:  Michal Dabski  |          Owner:  nobody
                   Type:  Uncategorized  |         Status:  new
              Component:  Uncategorized  |        Version:  1.11
               Severity:  Normal         |       Keywords:
           Triage Stage:  Unreviewed     |      Has patch:  0
    Needs documentation:  0              |    Needs tests:  0
Patch needs improvement:  0              |  Easy pickings:  0
                  UI/UX:  0              |
-----------------------------------------+------------------------
 Consider the sample model with a mixin class:
 {{{
 from django.db import models

 class TestMixin(object):
     pass


 class TestModel(TestMixin, models.Model):
     sample_field = models.CharField(max_length=20)
 }}}

 When running makemigrations, django creates a perfectly good and valid
 migration:
 {{{
 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.3 on 2017-07-26 17:03
 from __future__ import unicode_literals

 from django.db import migrations, models
 import migration_test.models


 class Migration(migrations.Migration):

     initial = True

     dependencies = [
     ]

     operations = [
         migrations.CreateModel(
             name='TestModel',
             fields=[
                 ('id', models.AutoField(auto_created=True,
 primary_key=True, serialize=False, verbose_name='ID')),
                 ('sample_field', models.CharField(max_length=20)),
             ],
             bases=(migration_test.models.TestMixin, models.Model),
         ),
     ]

 }}}
 SQL:

 {{{
 BEGIN;
 --
 -- Create model TestModel
 --
 CREATE TABLE "migration_test_testmodel" ("id" serial NOT NULL PRIMARY KEY,
 "sample_field" varchar(20) NOT NULL);
 COMMIT;
 }}}



 Next, refactor mixin class to add a field to it - need to change mixin's
 base class to {{{models.Model}}}, otherwise field will not be correctly
 inherited by models:
 {{{
 class TestMixin(models.Model):
     mixin_field = models.CharField(max_length=20, default='test')

     class Meta:
         abstract = True
 }}}

 Thie creates a new migration which adds {{{mixin_field}}} to it - nothing
 special. However, when applying both migrations after the model changes,
 second migration fails with the following error
 {{{django.db.utils.ProgrammingError: column "mixin_field" of relation
 "migration_test_testmodel" already exists}}}. as it turns out, the first
 migration's SQL has now changed to include {{{mixin_field}}}:
 {{{
 BEGIN;
 --
 -- Create model TestModel
 --
 CREATE TABLE "migration_test_testmodel" ("id" serial NOT NULL PRIMARY KEY,
 "mixin_field" varchar(20) NOT NULL, "sample_field" varchar(20) NOT NULL);
 COMMIT;
 }}}
 The python code of the migration obviously has not changed, but the
 resulting SQL did, and it includes a field not explicitly listed in the
 migration's {{{fields}}}.

 Note:
 - this does not happen if the mixin class extends {{{models.Model}}} to
 begin with.
 - if model extends a mixin that extends {{{object}}}, it ends up in
 model's {{{bases}}}, however if mixin extends {{{model.Model}}} it does
 not.



 Tested with Django 1.11.3, Python 2.7

-- 
Ticket URL: <https://code.djangoproject.com/ticket/28438>
Django <https://code.djangoproject.com/>
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