Question

I want to store blog and its tags as separate documents.

Blog post should have tag_ids field and tag shouldn’t have blog_posts_ids field.

Mongoid provides many to many relations out of the box, but it requires both documents of many to many relation to have _ids field.

class BlogPost
  include Mongoid::Document
  field :title
  references_many :tags, :stored_as => :array, :inverse_of => :blog_posts
end

class Tag
  include Mongoid::Document
  field :name
  # I DON'T WANT TO STORE BLOG_POSTS_IDS IN TAG DOCUMENT
  references_many :blog_posts, :stored_as => :array, :inverse_of => :tags
end

Answer 1

You can get around it with a method on Tag to fake the other side of the association

class BlogPost
  include Mongoid::Document
  field :title
  references_many :tags, :stored_as => :array, :inverse_of => :blog_posts
end

class Tag
  include Mongoid::Document
  field :name

  def blog_posts
    # will match all BlogPost records where the tag_ids array contains self.id
    BlogPost.where(:tag_ids => self.id)
  end
end

Obviously, this isn’t as full-featured as :references_many , but you can similarly fake other aspects of the many-to-many relation. For example if you want the ability to assign a new blog_post to a tag you can add a simple create_blog_post method to Tag.

For many real-world situations, this kind of approach is practical as long as you keep the methods simple and don’t get carried away.