has_one through and polymorphic associations over multi-table inheritance

In the project i'm currently developing under rails 4.0.0beta1, i had the need for a user based authentication in which each user could be linked to an entity. I'm kinda new to rails and had some troubles doing so.

The model is as following:

class User < ActiveRecord::Base
end

class Agency < ActiveRecord::Base
end

class Client < ActiveRecord::Base
  belongs_to :agency
end

What i need is for a user to be able to link to either an agency or a client but not both (those two are what i'll be calling entities). It can have no link at all and at most one link.

First thing i looked for was how to do Mutli-Table inheritance (MTI) in rails. But some things blocked me:

• it was not available out of the box
• MTI looked kinda hard to implement for a newbie such as me
• the gems implementing the solutions seemed old and either too complexe or not complete
• the gems would have probably broke under rails4 as they had not been updated for a while

So i looked for another solution and i found polymorphic associations.

I've be on this since yesterday and took some time to make it work even with the help of Rails polymorphic has_many :through and ActiveRecord, has_many :through, and Polymorphic Associations

I managed to make the examples from the question above work but it took a while and i finally have two problems:

1. How to transform the relations in user into a has_one association and be able to access "blindly" the linked entity ?
2. How to set a constraint so that no user can have more than one entity ?
3. Is there a better way to do what i want ?

Here's a fully working example:

The migration file:

class CreateUserEntities < ActiveRecord::Migration
  def change
    create_table :user_entities do |t|
      t.integer :user_id
      t.references :entity, polymorphic: true

      t.timestamps
    end

    add_index :user_entities, [:user_id, :entity_id, :entity_type]
  end
end

The models:

class User < ActiveRecord::Base
  has_one :user_entity

  has_one :client, through: :user_entity, source: :entity, source_type: 'Client'
  has_one :agency, through: :user_entity, source: :entity, source_type: 'Agency'

  def entity
    self.user_entity.try(:entity)
  end

  def entity=(newEntity)
    self.build_user_entity(entity: newEntity)
  end
end

class UserEntity < ActiveRecord::Base
  belongs_to :user
  belongs_to :entity, polymorphic: true

  validates_uniqueness_of :user
end

class Client < ActiveRecord::Base
  has_many :user_entities, as: :entity
  has_many :users, through: :user_entities
end

class Agency < ActiveRecord::Base
  has_many :user_entities, as: :entity
  has_many :users, through: :user_entities
end

As you can see i added a getter and a setter that i named "entity". That's because has_one :entity, through: :user_entity raises the following error:

ActiveRecord::HasManyThroughAssociationPolymorphicSourceError: Cannot have a has_many :through association 'User#entity' on the polymorphic object 'Entity#entity' without 'source_type'. Try adding 'source_type: "Entity"' to 'has_many :through' definition.

Finally, here are the tests i set up. I give them so that everyone understands know ho you can set and access data between those objects. i won't be detailing my FactoryGirl models but they're pretty obvious

require 'test_helper'

class UserEntityTest < ActiveSupport::TestCase

  test "access entity from user" do
    usr = FactoryGirl.create(:user_with_client)

    assert_instance_of client, usr.user_entity.entity
    assert_instance_of client, usr.entity
    assert_instance_of client, usr.client
  end

  test "only right entity is set" do
    usr = FactoryGirl.create(:user_with_client)

    assert_instance_of client, usr.client
    assert_nil usr.agency
  end

  test "add entity to user using the blind rails method" do
    usr = FactoryGirl.create(:user)
    client = FactoryGirl.create(:client)

    usr.build_user_entity(entity: client)
    usr.save!

    result = UserEntity.where(user_id: usr.id)
    assert_equal 1, result.size
    assert_equal client.id, result.first.entity_id
  end

  test "add entity to user using setter" do
    usr = FactoryGirl.create(:user)
    client = FactoryGirl.create(:client)

    usr.client = client
    usr.save!

    result = UserEntity.where(user_id: usr.id)
    assert_equal 1, result.size
    assert_equal client.id, result.first.entity_id
  end

  test "add entity to user using blind setter" do
    usr = FactoryGirl.create(:user)
    client = FactoryGirl.create(:client)

    usr.entity = client
    usr.save!

    result = UserEntity.where(user_id: usr.id)
    assert_equal 1, result.size
    assert_equal client.id, result.first.entity_id
  end

  test "add user to entity" do
    usr = FactoryGirl.create(:user)
    client = FactoryGirl.create(:client)

    client.users << usr

    result = UserEntity.where(entity_id: client.id, entity_type: 'client')

    assert_equal 1, result.size
    assert_equal usr.id, result.first.user_id
  end

  test "only one entity by user" do

    usr = FactoryGirl.create(:user)
    client = FactoryGirl.create(:client)
    agency = FactoryGirl.create(:agency)

    usr.agency = agency
    usr.client = client
    usr.save!

    result = UserEntity.where(user_id: usr.id)
    assert_equal 1, result.size
    assert_equal client.id, result.first.entity_id

  end

  test "user uniqueness" do

    usr = FactoryGirl.create(:user)
    client = FactoryGirl.create(:client)
    agency = FactoryGirl.create(:agency)

    UserEntity.create!(user: usr, entity: client)

    assert_raise(ActiveRecord::RecordInvalid) {
      UserEntity.create!(user: usr, entity: agency)
    }

  end

end

I Hope this can be of some help to someone. I decided to put the whole solution here cause it seems to me like a good one compared to MTI and i think it shouldn't take someone that much time to set something like that up.