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
26090	Changing a ForeignKey to a OneToOne field leaves an extraneous index behind	Aymeric Augustin	nobody	"(This happened to me on PostgreSQL, I don't know about other databases.)

Assume a Child model that has a foreign key to its a Parent. `makemigrations` + `migrate` generate the following index:

{{{
CREATE INDEX app_child_<hash> ON app_child USING btree (parent_id);
}}}

If the foreign key is later turned into a one-to-one relationship, `makemigrations` + `migrate` generates the following constraint:

{{{
ALTER TABLE ONLY app_child
    ADD CONSTRAINT app_child_parent_id_key UNIQUE (parent_id);
}}}

The original index remains.

However, if the parent relationship had been originally declared as a one-to-one, only the constraint would have been generated, not the index. (I noticed after recreating all migrations from scratch for performance reasons and comparing the database schema.)

I believe that:

- this falls within use cases where the migrations framework should keep the database state consistent
- the index is redundant with the constraint and should have been dropped in that case"	Bug	closed	Migrations	1.9	Normal	fixed		jon.dufresne@…	Accepted	1	0	0	0	0	0