Rails idiom to avoid duplicates in has_many :through

Question

I have a standard many-to-many relationship between users and roles in my Rails app:

class User < ActiveRecord::Base
  has_many :user_roles
  has_many :ro         


        

Answer 1

i think the proper validation rule is in your users_roles join model:

validates_uniqueness_of :user_id, :scope => [:role_id]

Answer 2

Perhaps it is possible to create the validation rule

validates_uniqueness_of :user_roles

then catch the validation exception and carry on gracefully. However, this feels really hacky and is very inelegant, if even possible.

Answer 3

As long as the appended role is an ActiveRecord object, what you are doing:

user.roles << role

Should de-duplicate automatically for :has_many associations.

For has_many :through, try:

class User
  has_many :roles, :through => :user_roles do
    def <<(new_item)
      super( Array(new_item) - proxy_association.owner.roles )
    end
  end
end

if super doesn't work, you may need to set up an alias_method_chain.

Answer 4

You can use a combination of validates_uniqueness_of and overriding << in the main model, though this will also catch any other validation errors in the join model.

validates_uniqueness_of :user_id, :scope => [:role_id]

class User
  has_many :roles, :through => :user_roles do
    def <<(*items)
      super(items) rescue ActiveRecord::RecordInvalid
    end
  end
end

Answer 5

I think you want to do something like:

user.roles.find_or_create_by(role_id: role.id) # saves association to database
user.roles.find_or_initialize_by(role_id: role.id) # builds association to be saved later

Answer 6

This will create only one association in the database even if called multiple times Refer rails guide.

user.roles=[Role.first]