How to add a down block for Sequel add_unique_constraint migration

Question

I have a Sequel migration in PostgreSQL, which works up, but not down:

Sequel.migration do
  change do
    alter_table(:files) do
      add_unique_constraint [:name, :folder]
    end
  end
end

When trying to migrate down from this block, Sequel's error message says this is an "irreversible migration" and suggests writing my "own down method".

How do I write the down method for this particular migration?

Answer 1

Per the docs:

Sequel.migration do
  up do
    alter_table(:files) do
      add_unique_constraint [:name, :folder]
    end
  end

  down do
    alter_table(:files) do
      drop_constraint(:your_constraint_name, :type=>:unique)
    end
  end
end

You will have to figure out the name of your uniqueness constraint. It should appear in your schema, and it should be something like index_files_on_name_and_folder.