Question

I am little bit stuck with the following problem. I have two models:

class Book < ActiveRecord:Base
  has_and_belongs_to_many :tags
end

class Tag < ActiveRecord:Base
  has_and_belongs_to_many :books
end

I have a list of specific tags that can but must not be used in the tags table:

tag1, tag2, tag3, tag4, tag5, …

Each new book can have several tags. As usual, relationships are stored in a join table “books_tags”.

How can I get a list of all tags that are at least related to one book?

Answer 1

You can use :joins as an option in your find call. E.g.

Tag.find(:all, :select => 'distinct tags.*', :joins => :books)

This will only find tags that have a book associated and the :select => 'distinct tags.*' ensures you only retrieve each tag once even if they are associated with multiple books.