Can't migrate Django models with foreign key constraints

Question

I have a model with a foreign key field. I want to remove the foreign key reference. However, migration fails because of the following error:

Cannot delete or update a parent row: a foreign key constraint fails

I understand what is going on, but I don't know how to resolve this properly with Django. Right now (since I'm at the beginning of my project), I go into the MySQL database manually and delete the tables and re-migrate as if it was the first migration.

Is there a way, using Django, to get around this foreign key constraint issue?

I'm using Django 1.7
Database backend is MySQL

EDIT - Models Before/After migrations

Before:

class Skills(models.Model):
  # fields here...

class Project(models.Model):
  skills = models.ForeignKey(Skills, verbose_name = "Required Skills", blank = True, null = True)

After:

class Skill(models.Model):
  # fields here...

class Project(models.Model):
  skills = models.ForeignKey(Skill, verbose_name = "Required Skills", blank = True, null = True)

I'm pretty sure all I've done is removed the "Plural" from the Skill model. The the makemigrations command works fine, but the migrate command fails with the above noted error.

EDIT 2

Hit the same error:

Cannot drop column 'skills_id': needed in a foreign key constraint 'projects_project_skills_id_4cc7e00883ac4de2_fk_projects_skill_id'

This time I dropped the field skill from model Project

This is an error you can expect when you are deleting or updating a row; not when you are changing database schema. Can you post your models (before and after migration)? — Selcuk
– Selcuk, Commented Mar 22, 2015 at 15:05

Liyosi · Accepted Answer · 2016-08-03 06:12:49Z

1

A little hack I used:

Add a migration operations that first alters the field to say IntegerField before other operations i.e

operations = [ migrations.AlterField( model_name='projects_project', name='skills', field=models.IntegerField(default=0) ), ..... other migration entries now after this. ]

answered Aug 3, 2016 at 6:12

Liyosi

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1 Comment

Tatsh Over a year ago

Your hack worked for me. I had this same issue adding a ForeignKey column. I used migrations.AddField(...UUIDField...) first for the field then put the AlterField after.

Brian Sanchez · Accepted Answer · 2022-11-05 13:03:50Z

0

This is my SOLUTION (do this manually first)

ALTER TABLE forms_formentry DROP FOREIGN KEY forms_formentry_form_id_d0f23912_fk_forms_form_page_ptr_id

This is the ERROR

django.db.utils.OperationalError: (1833, "Cannot change column 'page_ptr_id': used in a foreign key constraint 'forms_formentry_form_id_d0f23912_fk_forms_form_page_ptr_id' of table 'dbname.forms_formentry'")

answered Nov 5, 2022 at 13:03

Brian Sanchez

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