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referencesを使用した外部キーに、あとからUnique属性を設定する

Last updated at Posted at 2021-11-11

Postテーブルを作成し、userテーブルを外部参照にした。
しかし、あとからuser_idをユニークにしたくなった。

xxxxxxxxxxxxxx_create_posts.rb
class CreatePosts < ActiveRecord::Migration[6.1]
  def change
    enable_extension 'pgcrypto' unless extension_enabled?('pgcrypto')
    create_table :posts, id: :uuid do |t|
      t.references :user, type: :uuid, null: false, foreign_key: true
      t.timestamps
    end
  end
end

※ポスグレで、idをuuidにしています。

あとからユニーク制約をつけたい

成功した例

indexを一旦削除してから、unique属性をつけたindexを追加する。

xxxxxxxxxxxxxx_add_index_user_id_to_posts.rb
class AddIndexUserIdToPosts < ActiveRecord::Migration[6.1]
  def change
    remove_index :posts, :user_id
    add_index :posts, :user_id, unique: true
  end
end

失敗した例

xxxxxxxxxxxxxx_add_index_user_id_to_posts.rb
class AddIndexUserIdToPosts < ActiveRecord::Migration[6.1]
  def change
    add_index :posts, :user_id, unique: true
  end
end

これだと、以下のように「もうindexがあるからダメだよ」と言われてしまう。

PG::DuplicateTable: ERROR:  relation "index_posts_on_user_id" already exists
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