Why are foreign keys rewritten when adding related_name?

[Hanne Moa]

I added "related_name" to an exiting ForeignKey and checked with
"django-admin sqlmigrate" what would be done.

It seems that the foreign key constraint is dropped, then the exact
same constraint is added back. Why?

Example: If we have the classes:

class Wall(models.Model):
..

class Door(models.Model):
..wall = models.ForeignKey(Wall)

and change Door.wall to models.ForeignKey(Wall, related_name='walls'), then:

Prior to this, there exist (postgres) the constraint:

"app_door_wall_id_45647_fk_app_wall" FOREIGN KEY (wall_id) REFERENCES
app_wall(id) DEFERRABLE INITIALLY DEFERRED.

"django_admin sqlmigrate" reports:

SET CONSTRAINTS "app_door_wall_id_45647_fk_app_wall" IMMEDIATE; ALTER
TABLE "app_door" DROP CONSTRAINT
"app_door_wall_id_45647_fk_app_wall";
ALTER TABLE "app_door" ADD CONSTRAINT
"app_door_wall_id_45647_fk_app_wall" FOREIGN KEY ("wall_id")
REFERENCES "app_wall" ("id") DEFERRABLE INITIALLY DEFERRED;

After running the migration, which is run as there's stuff in the log,
there exists seemingly the same constraint as before:

"app_door_wall_id_45647_fk_app_wall" FOREIGN KEY (wall_id) REFERENCES
app_wall(id) DEFERRABLE INITIALLY DEFERRED.

Why waste a connection this way?

--
HM

[Simon Charette]

Hello HM,

I know that some changes have been made to avoid unnecessary foreign key

rebuilds on some option changes.

Are you experiencing this on 2.1 and 2.2 pre release?

Simon

[Hanne Moa]

No, 1.11. This project is still on Python 2.7 but I do think we'll
have it on Python 3(.5) before the year is over.

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