Modify foreign key in Ecto

Question

I have this original migration that have already been run and sent upstream:

create table(:videos) do
  add :url, :string
  add :title, :string
  add :description, :text
  add :user_id, references(:users, on_delete: :nothing)

  timestamps
end
create index(:videos, [:user_id])

Now i wish to change the foreign key on user_id to cascade deletions, so that when a user is deleted all of his associated videos will also be deleted.

I have tried the following migration:

alter table(:videos) do
  modify :user_id, references(:users, on_delete: :delete_all)
end

But this raises an error:

(Postgrex.Error) ERROR (duplicate_object): constraint "videos_user_id_fkey" for relation "videos" already exists

How can I formulate a migration script that will change this foreign key according to my requirement?

---

UPDATE

I ended up with the following solution:

def up do
  execute "ALTER TABLE videos DROP CONSTRAINT videos_user_id_fkey"
  alter table(:videos) do
    modify :user_id, references(:users, on_delete: :delete_all)
  end
end

def down do
  execute "ALTER TABLE videos DROP CONSTRAINT videos_user_id_fkey"
  alter table(:videos) do
    modify :user_id, references(:users, on_delete: :nothing)
  end
end

this drops the constraint before ecto tries to recreate it.

Answer 1

You can drop the index before calling alter:

drop_if_exists index(:videos, [:user_id])
alter table(:videos) do
  modify :user_id, references(:users, on_delete: :delete_all)
end

Doing the opposite is a little trickier:

execute "ALTER TABLE videos DROP CONSTRAINT videos_user_id_fkey"
create_if_not_exists index(:videos, [:user_id])