Polymorphic habtm relationships with Rails/ActiveRecord

Question

How would I go about creating a polymorphic has_and_belongs_to_many relationship with Rails/ActiveRecord?

Most of the examples I see involve creating a belongs_to relationship which limits my polymorphic-side to being related to only one parent:

Table: Task

Table: Tasks_Targets

Table: CustomerStore

Table: SoftwareSystem

Both CustomerStore and SoftwareSystem would be of type "Targetable" in this circumstance. From what I understand, if I implement the polymorphic relationship as most examples show, I'd only be able to relate a Targetable to a Task once.

Some clarification might help as most searches online still leave some of the theory behind this relationship unexplained...

Thanks!

Answer 1

Given your explanation of your domain, I've whipped up a small test-driven example of how you might solve your problem. If you see any domain inconsistencies, please feel free to clarify further (I'm using my acts_as_fu gem to whip up test models on the fly).

require 'acts_as_fu'

# class Task < ActiveRecord::Base
build_model(:tasks) do
  integer :task_target_id

  has_many :task_targets
  has_many :customer_stores, :through => :task_targets, :source => :targetable, :source_type => 'CustomerStore'
  has_many :software_systems, :through => :task_targets, :source => :targetable, :source_type => 'SoftwareSystem'
end

# class TaskTarget < ActiveRecord::Base
build_model(:task_targets) do
  string  :targetable_type
  integer :targetable_id
  integer :task_id

  belongs_to :targetable, :polymorphic => true
  belongs_to :task
end

# class CustomerStore < ActiveRecord::Base
build_model(:customer_stores) do
  has_many :task_targets, :as => :targetable
  has_many :tasks, :through => :task_targets
end

# class SoftwareSystem < ActiveRecord::Base
build_model(:software_systems) do
  has_many :task_targets, :as => :targetable
  has_many :tasks, :through => :task_targets
end

require 'test/unit'

class PolymorphicDomainTest < Test::Unit::TestCase
  # Test that customer stores can have multiple tasks
  def test_customer_store_gets_task
    task = Task.create!
    customer_store = CustomerStore.create!
    customer_store.task_targets.create! :task => task
    assert customer_store.tasks.include?(task)
  end

  def test_many_customer_stores_get_task
    task_a = Task.create!
    task_b = Task.create!
    customer_store = CustomerStore.create! :tasks => [task_a, task_b]
    assert customer_store.tasks.include?(task_a)
    assert customer_store.tasks.include?(task_b)
  end

  # Test that software systems can have multiple tasks
  def test_software_system_gets_task
    task = Task.create!
    software_system = SoftwareSystem.create!
    software_system.task_targets.create! :task => task
    assert software_system.tasks.include?(task)
  end

  def test_many_software_systems_get_task
    task_a = Task.create!
    task_b = Task.create!
    software_system = SoftwareSystem.create! :tasks => [task_a, task_b]
    assert software_system.tasks.include?(task_a)
    assert software_system.tasks.include?(task_b)
  end

  # Test that Tasks can have multiple customer stores
  def test_task_has_many_customer_stores
    task = Task.create!
    customer_store_a = CustomerStore.create!
    customer_store_b = CustomerStore.create!
    task.customer_stores = [customer_store_a, customer_store_b]
    task.save!
    task.reload
    assert task.customer_stores.include?(customer_store_a)
    assert task.customer_stores.include?(customer_store_b)
  end

  # Test that Tasks can have multiple software systems
  def test_task_has_many_software_systems
    task = Task.create!
    software_system_a = SoftwareSystem.create!
    software_system_b = SoftwareSystem.create!
    task.software_systems = [software_system_a, software_system_b]
    task.save!
    task.reload
    assert task.software_systems.include?(software_system_a)
    assert task.software_systems.include?(software_system_b)
  end
end

Answer 2

To complement nakajima's answer with regards to your concern, here's how I would do it:

class Task < ActiveRecord::Base
  def targets
    # Get Array of all targetables
    tt = TaskTarget.select_all("SELECT targetable_type, targetable_id FROM task_targerts WHERE task_id = #{self[:id]}")

    # Build Hash of targetable_type => Array of targetable_ids
    targetables = Hash.new { |hash, key| hash[key] = [] }
    tt.each do |targetable|
      targetables[targetable.targetable_type] << targetable.targetable_id
    end

    # Query each "targetable" table once and merge all results
    targetables.keys.map{|key| (eval key).find(targetables[key])}.flatten
  end
end

Make sure to index task_id in table task_targets.