Rails 8 Adds if_not_exists Option On The add_enum_value.

Rails 7 added support for custom enum types in PostgreSQL with introduction of create_enum to add a new enum type and t.enum to add a column.

Rails 7.1 has extended the ability to rename an enum, add enum value and rename enum value for the Postgres database adapter.

The add_enum_value method provides a straightforward way to add new values to a PostgreSQL enum type without needing to execute raw SQL.

class AddEnumToArticles < ActiveRecord::Migration[7.2]
  def change
    create_enum :status, ["draft", "published", "archived", "trashed"]

    safety_assured do
      change_table :articles do |t|
        t.enum :current_status, enum_type: "status", default: "draft", null: false
      end
    end
  end
end

class AddReviewToArticleStatus < ActiveRecord::Migration[7.2]
  def change
    add_enum_value :status, 'review'
  end
end

Before

When we use add_enum_value, PostgreSQL checks for duplicates and raises a PG::DuplicateObject error if a value already exists. ActiveRecord captures this as ActiveRecord::StatementInvalid when trying to add a duplicate value.

class AddReviewToArticleStatus < ActiveRecord::Migration[7.2]
  def change
    add_enum_value :status, 'review'
  end
end

PG::DuplicateObject: ERROR:  enum label "review" already exists

ActiveRecord::StatementInvalid: PG::DuplicateObject: ERROR:  enum label "review" already exists (ActiveRecord::StatementInvalid)

To avoid the adding of dubplicate enums, we can use PostgreSQL IF NOT EXISTS but it only works with raw SQL. So we can not use it with add_enum_value.

After

Rails 8 adds support for PostgreSQL IF NOT EXISTS functionality with the introduction of if_not_exists option on the add_enum_value method.

With this if_not_exists option, we can now safely add a new enum value, preventing errors if the value already exists.

class AddReviewToArticleStatus < ActiveRecord::Migration[8.0]
  def change
    add_enum_value :status, 'review', if_not_exists: true
  end
end

If the enum doesn’t exist then the above migration will add the enum otherwise it won’t raise an error.