Error when running rails db:rollback that would result in dropping an index

[wolfgangrittner]

Given this migration:

class CreateFoos < ActiveRecord::Migration[5.2]
  def change
    create_table :foos do |t|
      t.bigint :number
      t.references :user

      t.timestamps
    end

    add_index :foos, [:user_id, :number], unique: true
  end
end

Steps to reproduce

• run rails db:migrate and then
• run rails db:rollback

Expected Result

No error messages and the table and the index gone from the database.

Actual Result

The rollback throws a database error, and the index and table are not dropped from the database.

# ...
PG::InternalError: ERROR:  index "index_foos_on_user_id_and_number" is in use as unique constraint (use CASCADE if you really want to drop it)
: DROP INDEX  "index_foos_on_user_id_and_number"
# ... 

workaround

I found a workaround for it, which works for postgresql and cockroachdb, but is no permanent solution. You need to change the migration to look like this, which will yield the expected result on rollback:

class CreateFoos < ActiveRecord::Migration[5.2]
  def up
    create_table :foos do |t|
      t.bigint :number
      t.references :user

      t.timestamps
    end

    add_index :foos, [:user_id, :number], unique: true
  end

  def down
    drop_table :foos
  end
end

Not sure if this is something the adapter should be concerned with, or if this is something cockroachdb itself should deal different with (ie. like Postgres)?