[FIXED] how to avoid duplicates in a has_many :through relationship?

Issue

How can I achieve the following? I have two models (blogs and readers) and a JOIN table that will allow me to have an N:M relationship between them:

class Blog < ActiveRecord::Base
  has_many :blogs_readers, :dependent => :destroy
  has_many :readers, :through => :blogs_readers
end

class Reader < ActiveRecord::Base
  has_many :blogs_readers, :dependent => :destroy
  has_many :blogs, :through => :blogs_readers
end

class BlogsReaders < ActiveRecord::Base
  belongs_to :blog
  belongs_to :reader
end

What I want to do now, is add readers to different blogs. The condition, though, is that I can only add a reader to a blog ONCE. So there mustn't be any duplicates (same readerID, same blogID) in the BlogsReaders table. How can I achieve this?

The second question is, how do I get a list of blog that the readers isn't subscribed to already (e.g. to fill a drop-down select list, which can then be used to add the reader to another blog)?

Solution

What about:

Blog.find(:all,
          :conditions => ['id NOT IN (?)', the_reader.blog_ids])

Rails takes care of the collection of ids for us with association methods! :)

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

Answered By - Josh Delsman

Answer Checked By - Senaida (PHPFixing Volunteer)