Example code to allow instances of the `User` model to have multiple `roles`

add_roles_to_users.rb

# Example code to allow instances of the `User` model to have multiple `roles`:


# Migration -------------------

class AddRolesToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :roles, :text, array: true, default: [], null: false
  end
end


# Model ----------------------

class User < ApplicationRecord
  VALID_ROLES = [
    "visitor",
    "registered",
    "pro",
    "premium",
    "admin",
  ].freeze

  # All the following methods are optional convenience methods!
  # None of them are necessary to make this work!
  # But they help to add or remove single roles to a user.

  after_initialize :define_has_role_methods

  def add_role!(role)
    add_role(role).save!
  end

  def add_role(role)
    unless VALID_ROLES.include?(role.to_s)
      errors.add(:roles, "invalid role, must be in #{VALID_ROLES}")
      return self
    end

    roles << role.to_s
    roles.uniq!

    self
  end

  def delete_role!(role)
    delete_role(role).save!
  end

  def delete_role(role)
    self.roles -= Array(role.to_s)
    self
  end

  private

  # optional convenience methods
  # defines visitor?, registered?, pro?, premium?, admin? ... query methods
  def define_has_role_methods
    VALID_ROLES.each do |role|
      self.class.send(:define_method, "#{role}?".to_sym) do
        roles.include?(role.to_s)
      end
    end
  end
end


# Factory ----------------------

FactoryBot.define do
  factory :user, class: "User" do
    roles { [User::VALID_ROLES.sample] }
  end
end


# Validator ---------------------

# optional to generation better error messages
# and let the user know exactly which elements are invalid
#
class ArrayInclusionValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    values = Array(value) || []
    rejected = []

    rejected.concat(values - options[:in]) if options[:in]
    rejected.concat(values.find_all { |v| !options[:proc].call(v) }) if options[:proc]

    return if rejected.blank?

    formatted_rejected = rejected.uniq.collect(&:inspect).join(", ")

    record.errors.add(
      attribute, :inclusion, **options.except(:in).merge!(rejected_values: formatted_rejected, value:)
    )
  end
end