Ruby-on-rails – ActiveRecord, has_many :through, and Polymorphic Associations

activerecordpolymorphic-associationsruby-on-rails

Folks,

Want to make sure I understand this correctly. And please disregard the case for inheritance here (SentientBeing), trying to instead focus on polymorphic models in has_many :through relationships. That said, consider the following…

class Widget < ActiveRecord::Base
  has_many :widget_groupings

  has_many :people, :through => :widget_groupings, :source => :person, :conditions => "widget_groupings.grouper_type = 'Person'"
  has_many :aliens, :through => :widget_groupings, :source => :alien, :conditions => "video_groupings.grouper_type = 'Alien'"
end

class Person < ActiveRecord::Base
  has_many :widget_groupings, :as => grouper
  has_many :widgets, :through => :widget_groupings
end

class Alien < ActiveRecord::Base
  has_many :widget_groupings, :as => grouper
  has_many :widgets, :through => :widget_groupings  
end

class WidgetGrouping < ActiveRecord::Base
  belongs_to :widget
  belongs_to :grouper, :polymorphic => true
end

In a perfect world, I'd like to, given a Widget and a Person, do something like:

widget.people << my_person

However, when I do this, I've noticed the 'type' of the 'grouper' is always null in widget_groupings. However, if I to something like the following:

widget.widget_groupings << WidgetGrouping.new({:widget => self, :person => my_person})

Then all works as I would have normally expected. I don't think I've ever seen this occur with non polymorphic associations and just wanted to know if this was something specific to this use case or if I'm potentially staring at a bug.

Thanks for any help!

Best Answer

There is a known issue with Rails 3.1.1 that breaks this functionality. If you are having this problem first try upgrading, it's been fixed in 3.1.2

You're so close. The problem is you're misusing the :source option. :source should points to the polymorphic belongs_to relationship. Then all you need to do is specify :source_type for the relationship you're trying to define.

This fix to the Widget model should allow you do exactly what you're looking for.

class Widget < ActiveRecord::Base
  has_many :widget_groupings

  has_many :people, :through => :widget_groupings, :source => :grouper, :source_type => 'Person'
  has_many :aliens, :through => :widget_groupings, :source => :grouper, :source_type => 'Alien'
end

Related Solutions

Ruby-on-rails – Eager loading of polymorphic associations in ActiveRecord

After digging through the Rails source I've discovered that you can force a join by referencing a table other than the current model in either the select, conditions or order clauses.

So, if I specify an order on the Asset table:

Image.first(
      :conditions => {:id => id},
      :include => :asset,
      :order => "asset.id"
)

The resulting SQL will use a left outer join and everything in one statement. I would have preferred an inner join, but I guess this will do for now.

Ruby-on-rails – has_one through and polymorphic associations over multi-table inheritance

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.

Best Answer

Related Solutions

Ruby-on-rails – Eager loading of polymorphic associations in ActiveRecord

Ruby-on-rails – has_one through and polymorphic associations over multi-table inheritance

Related Topic