has_many through additional attributes

Question

How do we set additional parameters in has_many through associations?

Thanks. Neelesh

Answer 1

Got the same problem here. Can't find any tutorials how to make it work on-the-fly in Rails 3. But you can still get what you want through the join model itself.

p = Post.new(:title => 'Post', :body => 'Lorem ipsum ...')
t = Tag.new(:title => 'Tag')

p.tags << t
p.save   # saves post, tag and also add record to posttags table, but additional attribute is NULL now
j = PostTag.find_by_post_id_and_tag_id(p,t)
j.user_id = params[:user_id]
j.save   # this finally saves additional attribute

Pretty ugly but this works from me.

Answer 2

Well I was in a similar situation where I wanted to have a join table that joined 3 models. But I wanted the third model ID to be gotten from the second model.

class Ingredient < ActiveRecord::Base

end

class Person < ActiveRecord::Base
  has_many :food
  has_many :ingredients_food_person
  has_many :ingredients, through: :ingredients_food_person
end

class Food
  belongs_to :person
  has_many :ingredient_food_person
  has_many :ingredients, through: :ingredients_food_person

  before_save do
    ingredients_food_person.each { |ifp| ifp.person_id = person_id }
  end
end

class IngredientFoodPerson < ActiveRecord::Base
  belongs_to :ingredient
  belongs_to :food
  belongs_to :person
end

And surprisingly, you can do something like this:

food = Food.new ingredients: [Ingredient.new, Ingredient.new]
food.ingredients_food_person.size # => 2
food.save

At first I thought that before I saved, I wouldn't have access to #ingredients_food_person after assigning #ingredients. But it generates the models automatically.

Answer 3

has_many :tags, :through => :post_tags, :conditions => ['tag.owner_id = ?' @owner.id]

Answer 4

This blog post has the perfect solution: http://www.tweetegy.com/2011/02/setting-join-table-attribute-has_many-through-association-in-rails-activerecord/

That solution is: create your ":through model" manually, rather than through the automated way when you append to its owner's array.

Using the example from that blog post. Where your models are:

class Product < ActiveRecord::Base
  has_many :collaborators
  has_many :users, :through => :collaborators
end

class User < ActiveRecord::Base
  has_many :collaborators
  has_many :products, :through => :collaborators
end

class Collaborator < ActiveRecord::Base
  belongs_to :product
  belongs_to :user
end

Previously you may have gone: product.collaborators << current_user.

However, to set the additional argument (in this example is_admin), rather than the automated way of appending to the array, you can do it manually like:

product.save && product.collaborators.create(:user => current_user, :is_admin => true)

This approach allows you to set the additional arguments at save-time. NB. the product.save is necessary if the model hasn't yet been saved, otherwise it can be omitted.