id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
34333	Django migrations adds constraint before adding field	Raphael Beekmann	nobody	"Hello,

I have a model, already created through previous migrations, and in a new migration I added a new field with an UniqueConstraint. The migrations script try to create the constraint first and then the new field, resulting an error : 

{{{
django.core.exceptions.FieldDoesNotExist: NewModel has no field named 'category'
}}}


To reproduce the bug : 

1. Create a project with two models linked together with a One-to-Many relation and an unique constraint : 

{{{
class Type(models.Model):
    name = models.CharField(max_length=10)


class Model(models.Model):
    name = models.CharField(max_length=10)
    type = models.ForeignKey(Type, on_delete=models.SET_NULL, null=True)
    date = models.DateField(auto_now=True)

    class Meta:
        constraints = (
            models.UniqueConstraint(fields=('date', 'type'), name='unique_type_for_date'),
        )
}}}

2. Create a migration file with `manage.py makemigrations`

3. Add a new model with another One-to-Many relation and unique constraint. The models looks like this : 


{{{
class Type(models.Model):
    name = models.CharField(max_length=10)


class Category(models.Model):
    name = models.CharField(max_length=10)


class Model(models.Model):
    name = models.CharField(max_length=10)
    type = models.ForeignKey(Type, on_delete=models.SET_NULL, null=True)
    category = models.ForeignKey(Category, on_delete=models.SET_NULL, null=True)
    date = models.DateField(auto_now=True)

    class Meta:
        constraints = (
            models.UniqueConstraint(fields=('date', 'type'), name='unique_type_for_date'),
            models.UniqueConstraint(fields=('date', 'category'), name='unique_category_for_date'),
        )
}}}


4. Create a new migration file. The order of the migration's steps are incorrect and the migration crash : 


{{{
class Migration(migrations.Migration):

    dependencies = [
        ('app', '0001_initial'),
    ]

    operations = [
        migrations.CreateModel(
            name='Category',
            fields=[
                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('name', models.CharField(max_length=10)),
            ],
        ),
        migrations.AddConstraint(
            model_name='model',
            constraint=models.UniqueConstraint(fields=('date', 'category'), name='unique_category_for_date'),
        ),
        migrations.AddField(
            model_name='model',
            name='category',
            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.SET_NULL, to='app.category'),
        ),
    ]
}}}

"	Bug	new	Migrations	4.1	Normal		migration, constraint, field		Unreviewed	0	0	0	0	0	0