Converting a boolean column to an enum in ActiveRecord/Rails

20211227213338_create_users.rb

# frozen_string_literal: true

# db/migrate/20211227213338_create_users.rb
#
# This is an example of what the Users table might have been created as
# or would look like before the migration from a boolean field to an enum
class CreateUsers < ActiveRecord::Migration[7.0]
  def change
    create_table :users do |t|
      t.string :name, null: false
      t.string :email, null: false, unique: true
      # Realistically, this would probably be "active" not status; you'll want to do
      # a rename at the appropriate time. Assuming all the code changes & migration
      # have been run before doing the actual conversion which is run in the other migration
      t.boolean :status, null: false, default: false, comment: 'true is active, false is inactive'

      t.timestamps
    end
  end
end

20211227213604_convert_user_status_to_enum.rb

# frozen_string_literal: true

# db/migrate/20211227213604_convert_user_status_to_enum.rb
#
# Scenario: 
# - You have a Users table with a boolean active column
# - You realize you need an extra "state" of pending
# - You want to convert your boolean to an enum & coerce the existing data
#
# Not in scope:
# - Renaming the original field from active -> status
class ConvertUserStatusToEnum < ActiveRecord::Migration[7.0]
  def up
    execute <<-SQL
      -- Creates a new postgres type named user_status_fields which allows
      -- for additional states aside from true/false
      CREATE TYPE user_status_fields AS ENUM ('active', 'inactive', 'pending')
    SQL

    change_column_default :users, :status, nil

    change_column :users, :status, <<~SQL
      user_status_fields
      USING
      (
        CASE status
        WHEN true THEN 'active'::user_status_fields
        WHEN false THEN 'inactive'::user_status_fields
        END
      )
    SQL

    change_column_comment :users, :status, from: 'true is active, false is inactive',
                                           to: 'user_status_fields enum value'

    change_column_default :users, :status, :inactive
  end

  def down
    change_column_default :users, :status, nil

    change_column :users, :status, <<~SQL
      boolean
      USING
      (
        CASE status
        WHEN 'active'::user_status_fields THEN true
        WHEN 'inactive'::user_status_fields THEN false
        -- You just have to decide what pending means when you roll back
        WHEN 'pending'::user_status_fields THEN false
        END
      )
    SQL

    change_column_default :users, :status, false

    change_column_comment :users, :status, from: 'user_status_fields enum value',
                                           to: 'true is active, false is inactive'

    execute <<-SQL
      DROP TYPE user_status_fields
    SQL
  end
end

user.rb

# frozen_string_literal: true

# app/models/user.rb
class User < ApplicationRecord
  # The keys must be strings, not symbols, for ActiveRecord to recognize
  # them. You get the standard scopes just like a regular enum, like 
  # User.active & User.pending
  enum status: { active: 'active', inactive: 'inactive', pending: 'pending' }
end