Changing a primary key doesn't change corresponding foreign keys

[Chen Chun-Chia]

Problem Description

Changes of primary key do not apply to all relative models when applying multiple migrations.

Django Version

Both 1.11.1 and 1.11.5 are tested

Reproduce Steps

1. Initial model

   class Pub(models.Model):
       code = models.CharField(max_length=4, primary_key=True)
   
   
   class Staff(models.Model):
       pub = models.ForeignKey('Pub')
   
   
   class Album(models.Model):
       pub = models.ForeignKey('Pub')
   
   
   class Music(models.Model):
       album = models.ForeignKey('Album')
       name = models.CharField(max_length=32)
   

2. Make migrations --> 0001_initial.py

3. Alter max_length of Music.name to 64 --> 0002_alter_music_name.py

4. Alter max_length of Pub.code to 8 --> 0003_alter_pub_code.py

5. Final model

   class Pub(models.Model):
       code = models.CharField(max_length=16, primary_key=True)  # 0003_alter_pub_code.py
   
   
   class Staff(models.Model):
       pub = models.ForeignKey('Pub')
   
   
   class Album(models.Model):
       pub = models.ForeignKey('Pub')
   
   
   class Music(models.Model):
       album = models.ForeignKey('Album')
       name = models.CharField(max_length=64)  # 0002_alter_music_name.py
   

6. Use clean database (drop and recreate database)

7. Run migrate

8. Result

   Length of Pub.code : 8
   Length of Staff.pub_id : 4  ### wrong
   Length of Album.pub_id : 8
   

Observation

After applying 0002_alter_music_name.py, only Album exists in Pub._meta.related_objects of state.apps, but Staff is gone.

[Tim Graham]

Duplicate of #25012.