Rails: is it possible to add extra attribute to a has_and_belongs_to_many association?

Question

What I mean is if I have two models, connected by a has_and_belongs_to_many association, can I store other data in the join table for each association? That is, the extra data would not be part of a single record in either table, but instead of the connection between them.

My actual models are as follows:

class Part < ActiveRecord::Base
  has_and_belongs_to_many :assemblies
  has_and_belongs_to_many :packages
  belongs_to :user

  validates :name, :user_id, :presence => true
end

class Package < ActiveRecord::Base
  has_and_belongs_to_many :parts
  belongs_to :user
end

So the point is that each part is available in many packages, and each package has different parts. What I want to add is a quantity. That would not be quantity of each part, but of each package of each part.

I can't find how to do this in ActiveRecord. If I was not using rails/activerecord, I'd just add a quantity column to the join table which relates parts to packages. I could obviously make this change in a migration, but how would I access the value using ActiveRecord?

Answer 1

Short answer no you cannot with a HABTM relationship. It is only intended for simple many to many relationships.

You will need to use a has_many :through relationship. In this scenario you will create a join model (PartPackage) where you can define the extra attributes that you need.

class Part < ActiveRecord::Base
  has_many :part_packages
  has_many :packages, :through => :part_packages

  has_and_belongs_to_many :assemblies
  belongs_to :user

  validates :name, :user_id, :presence => true
end

class PartPackage < ActiveRecord::Base
  belongs_to :part
  belongs_to :package
end

class Package < ActiveRecord::Base
  has_many :part_packages
  has_many :parts, :through => :part_packages
  belongs_to :user
end

Answer 2

There is a key difference between has_many :through and has_and_belongs_to_many the Rails guide explains the differences between the two options in greater detail, however if you want to add data that describes the relationship then use has_many :through and you can access the model that joins the two.

This is what has_many :through looks like:

Answer 3

Actually, this is possible.

1) Add a quantity key to the packages_parts join table

2) Add this to your Part model

has_and_belongs_to_many :packages, -> { select("packages.*, 
packages_parts.quantity") }

Now you can do part.packages[0].quantity, for example.

What this does is tell Rails to fetch the quantity key whenever it gets the packages for a specific part.

(You can do the same for your Package model if you want to do package.parts.first.quantity as well - just put this on the Package model also, replacing 'packages' with 'parts')

More info: https://ducktypelabs.com/using-scope-with-associations/