# Gemfile
gem "tag_columns"

# db/migrate/TIMESTAMP_add_groups_to_user.rb
class AddGroupsToUser < ActiveRecord::Migration[5.0]
  def change
    add_column :users, :groups, :string, array: true, default: [], null: false
    add_index :users, :groups, using: "gin"
  end
end

# app/models/user.rb
class User < ApplicationRecord
  include TagColumns
  tag_columns :groups
end

user = User.find(1)

# assigning tags
user.groups << :reader
user.groups << :writer
user.save

# checking tags
is_writer            = user.has_group?(:writer)
is_reader_or_writer  = user.has_any_groups?(:reader, :writer)
is_reader_and_writer = user.has_all_groups?(:reader, :writer)

# finding tagged records
assigned                = User.with_groups
unassigned              = User.without_groups
writers                 = User.with_any_groups(:writer)
non_writers             = User.without_any_groups(:writer)
readers_or_writers      = User.with_any_groups(:reader, :writer)
readers_and_writers     = User.with_all_groups(:reader, :writer)
non_readers_and_writers = User.without_all_groups(:reader, :writer)

# find unique tags across all users
User.unique_groups

# find unique tags for users with the last name 'Smith'
User.unique_groups(last_name: "Smith")