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
31788	Models migration with change field foreign to many and deleting unique together.	budzichd	David Wobrock	"I have models like

{{{#!python
class Authors(models.Model):
    project_data_set = models.ForeignKey(
        ProjectDataSet,
        on_delete=models.PROTECT
    )
    state = models.IntegerField()
    start_date = models.DateField()

    class Meta:
         unique_together = (('project_data_set', 'state', 'start_date'),)
}}}

and

{{{
class DataSet(models.Model):
    name = models.TextField(max_length=50)


class Project(models.Model):
    data_sets = models.ManyToManyField(
        DataSet,
        through='ProjectDataSet',
    )
    name = models.TextField(max_length=50)


class ProjectDataSet(models.Model):
    """"""
    Cross table of data set and project
    """"""
    data_set = models.ForeignKey(DataSet, on_delete=models.PROTECT)
    project = models.ForeignKey(Project, on_delete=models.PROTECT)

    class Meta:
        unique_together = (('data_set', 'project'),)
}}}

when i want to change field project_data_set in Authors model from foreign key field to many to many field I must delete a unique_together, cause it can't be on many to many field.
Then my model should be like:

{{{
class Authors(models.Model):

    project_data_set = models.ManyToManyField(
        ProjectDataSet,
    )

    state = models.IntegerField()
    start_date = models.DateField()
}}}

But when I want to do a migrations.
1. python3 manage.py makemigrations
2. python3 manage.py migrate
I have error:
{{{ValueError: Found wrong number (0) of constraints for app_authors(project_data_set, state, start_date)}}}
The database is on production, so I can't delete previous initial migrations, and this error isn't depending on database, cause I delete it and error is still the same.
My solve is to first delete unique_together, then do a makemigrations and then migrate. After that change the field from foreign key to many to many field, then do a makemigrations and then migrate.
But in this way I have 2 migrations instead of one.

I added attachment with this project, download it and then do makemigrations and then migrate to see this error.
 "	Bug	closed	Migrations	3.0	Normal	fixed		Markus Holtermann Simon Charette David Wobrock	Ready for checkin	1	0	0	0	0	0