Use Association Extensions to Build Join Attributes on a HMT

Apr 3, 2012

It’s common in Rails to use a has_many :through relationship to model User/Group Memberships. Sometimes we have extra data in the join that we would like to make use of, but getting that data in there can be combersome depending on our approach. For example, given the following diagram and schema:

ActiveRecord::Schema.define(:version => 20120324170519) do
  create_table "groups", :force => true do |t|
    t.string   "name"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

  create_table "memberships", :force => true do |t|
    t.integer  "user_id"
    t.integer  "group_id"
    t.string   "role"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

  create_table "users", :force => true do |t|
    t.string   "name"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end
end

We might deal directly with the join table to assign our additonal data.

@user = User.create(name: 'User 1')
@user = Group.create(name: 'Group 1')
@membership = Membership.create do |m|
  m.user = @user
  m.group = @group
  m.role = 'admin'
end
@user.admin? # => true
@user.editor? # => false

There’s a better way to pull this off …

@group.admins << @user
@user.admin? # => true
@user.editor? # => false

And this is how it’s done …

class User < ActiveRecord::Base
  has_many :memberships
  has_many :groups, :through => :memberships

  def admin?
    memberships.where(:role => 'admin').first
  end

  def editor?
    memberships.where(:role => 'editor').first
  end
end

class Membership < ActiveRecord::Base
  belongs_to :group
  belongs_to :user
end

class Group < ActiveRecord::Base
  has_many :memberships
  has_many :users, :through => :memberships

  has_many :admins, :through => :memberships, :source => :user,
    :conditions => "memberships.role = 'admin'" do
      def <<(admin)
        proxy_association.owner.memberships.create(:role => 'admin', :user => admin)
      end
  end

  has_many :editors, :through => :memberships, :source => :user,
    :conditions => "memberships.role = 'editor'" do
      def <<(editor)
        proxy_association.owner.memberships.create(:role => 'editor', :user => editor)
      end
  end
end

We’re defining an extension on our group’s has_many association which overrides the << method on that collection. We then tell the proxy association’s owner (which is our group object) to create the user/group join record, but with an additional role assignment of ‘admin’.

@group.admins << @user
@user.admin? # => true
@user.editor? # => false

Pretty expressive, thanks to ActiveRecord!

require 'test_helper'

class GroupTest < ActiveSupport::TestCase
  setup do
    @user_1 = User.create(name: 'User 1')
    @user_2 = User.create(name: 'User 2')
    @user_3 = User.create(name: 'User 3')
    @group = Group.create(name: 'Group 1')
  end

  test "No Memberships" do
    assert_equal @user_1.memberships.count, 0
  end

  test "@group.users << @user_1 sets nil role on membership" do
    @group.users << @user_1
    assert_equal @user_1.memberships.count, 1
    assert_equal @user_1.memberships.first.role, nil
  end

  test "@group.admins << @user_2 sets 'admin' role on membership" do
    @group.admins << @user_2
    assert_equal @user_2.memberships.count, 1
    assert_equal @user_2.memberships.first.role, 'admin'
  end

  test "@group.editors << @user_3 sets 'editor' role on membership" do
    @group.editors << @user_3
    assert_equal @user_3.memberships.count, 1
    assert_equal @user_3.memberships.first.role, 'editor'
  end

  teardown do
    User.delete_all
    Group.delete_all
    Membership.delete_all
  end
end