Modify foreign key in Ecto

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

Copied from the question.

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])

I'm not sure when this was added to Ecto, but at least in 2.1.6 there's no need for raw SQL anymore. drop/1 now supports constraints (drop_if_exists/1 doesn't though):

def up do
  drop constraint(:videos, "videos_user_id_fkey")
  alter table(:videos) do
    modify :user_id, references(:users, on_delete: :delete_all)
  end
end

def down do
  drop constraint(:videos, "videos_user_id_fkey")
  alter table(:videos) do
    modify :user_id, references(:users, on_delete: :nothing)
  end
end

In Ecto SQL 3.4.3:

"If the :from value is a %Reference{}, the adapter will try to drop the corresponding foreign key constraints before modifying the type."

modify :user_id, references(:users, on_delete: :delete_all), from: references(:users)

Should work. In working on a rollback, I worked out that this worked to clear the FK and remove the column:

remove :user_id, references(:users)