Setting db_constraint=False to an existing column does not properly drop the constraint

[Scott Stafford]

Repro steps, using PostgreSQL and Django 2.2.4:

1. Create a new model with a normal, constrained ForeignKey, such as:

class Child(models.Model):
    parent = models.ForeignKey('myapp.Parent', related_name="+", on_delete=models.DO_NOTHING)

2. Create a migration: python manage.py makemigrations

3. Add db_constraint=False, like so:

class Child(models.Model):
    parent = models.ForeignKey('myapp.Parent', related_name="+", on_delete=models.DO_NOTHING, db_constraint=False)

4. Create a second migration: python manage.py makemigrations

5. Peep at the generated SQL. The first migration creates the constraint, and the second fails to remove it:

(.env) c:\wc\dbconstraintnotdropped>python manage.py sqlmigrate myapp 0001
BEGIN;
--
-- Create model Parent
--
CREATE TABLE "myapp_parent" ("id" serial NOT NULL PRIMARY KEY);
--
-- Create model Child
--
CREATE TABLE "myapp_child" ("id" serial NOT NULL PRIMARY KEY, "parent_id" integer NOT NULL);
ALTER TABLE "myapp_child" ADD CONSTRAINT "myapp_child_parent_id_af46d0ab_fk_myapp_parent_id" FOREIGN KEY ("parent_id") REFERENCES "myapp_parent" ("id") DEFERRABLE INITIALLY DEFERRED;
CREATE INDEX "myapp_child_parent_id_af46d0ab" ON "myapp_child" ("parent_id");
COMMIT;

(.env) c:\wc\dbconstraintnotdropped>python manage.py sqlmigrate myapp 0002
BEGIN;
--
-- Alter field submission on child
--
COMMIT;