Rails 5: How can I change an existing model's ID type to UUID?

Question

I'm trying to change a Devise User model's id type to a uuid.

My migration looks like this:

class ChangeUserIdTypeToUuid < ActiveRecord::Migration[5.2]

  def up
    change_column :users, :id, :uuid
  end

  def down
    change_column :users, :id, :integer
  end
end

But when I run the migration I get an error:

== 20180909205634 ChangeUserIdTypeToUuid: migrating ===========================
-- change_column(:users, :id, :uuid)
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::DatatypeMismatch: ERROR:  column "id" cannot be cast automatically to type uuid
HINT:  You might need to specify "USING id::uuid".
: ALTER TABLE "users" ALTER COLUMN "id" TYPE uuid

There's a hint in there but I don't know what it's suggesting I do. It's not this:

change_column :users, :id, id::uuid

Why is the migration failing? What is the hint suggesting? How do I change the ID type to UUID?

Answer 1

Referring to the post this was suggested to be a duplicate of, I have managed to change the id type to uuid like so:

class ChangeUserIdTypeToUuid < ActiveRecord::Migration[5.2]

  def change
    add_column :users, :uuid, :uuid, default: "gen_random_uuid()", null: false

    change_table :users do |t|
      t.remove :id
      t.rename :uuid, :id
    end
    execute "ALTER TABLE users ADD PRIMARY KEY (id);"
  end
end

The difference from the best answer in the linked question is the way the UUID is generated. I'm using postgresql as my development db so I'm using gen_random_uuid() which is provided by the pgcrypto extension which I had previously enabled in preparation of using UUIDs in another model:

class EnablePgcryptoExtension < ActiveRecord::Migration[5.2]
  def change
    enable_extension 'pgcrypto'
  end
end