Question

Hi I have a posts model and a collections model, joined together by a collectionship model. When a user makes a post he adds the post to a collection, such as ‘music’. However, when I list all the user’s collections there are multiple ‘music’ entries for each post made instead of just 1.
I’m grabbing the collections with @collections = @user.posts.map(&:collections).flatten, if I add a .uniq on the end get no duplicates (@collections = @user.posts.map(&:collections).flatten.uniq) But could someone explain why I’d have to do this??? Thanks a lot.

UsersController

  def show
    @user = User.find(params[:id]) rescue nil
    @posts = @user.posts.paginate(:per_page => "10",:page => params[:page])
    @title = @user.name
    @collections = @user.posts.map(&:collections).flatten
  end

views/users/show.html.erb

 <h1>Collections</h1>

  <% @collections.each do |collection| %>
    <%= link_to collection.name, user_collection_posts_path(@user, link_name(collection)) %><br />
  <% end %>

collection model

class Collection < ActiveRecord::Base
  mount_uploader :image, CollectionUploader
  attr_accessible :name, :image, :user_id
  has_many :collectionships
  has_many :users, :through => :posts
  has_many :posts, :through => :collectionships
end

collectionship model

class Collectionship < ActiveRecord::Base
  belongs_to :post
  belongs_to :collection
  has_one :user, :through => :post
  attr_accessible :post_id, :collection_id
end

post model

  belongs_to :user
  has_many :collectionships
  has_many :collections, :through => :collectionships

user mdoel

has_many :posts, :dependent => :destroy
has_many :collections, :through => :posts

Answer 1

You’ve got the line that’s causing it. Here’s my take on why you’re seeing what you do (just an expansion of each step of evaluation of that line):

@user.posts #=>
[
    <Post1:
        id: 1492
        collections: ['history', 'spain']
    >,
    <Post2:
        id: 1912
        collections: ['history', 'shipwrecks']
    >
]

@user.posts.map(&:collections) #=>
[
    ['history', 'spain'],
    ['history', 'shipwrecks']
]

@user.posts.map(&:collections).flatten #=>
[
    'history',
    'spain',
    'history',
    'shipwrecks'
]

So you can see that each for each post, post.collections returns all the collections that post is in (as it should). And the flatten method doesn’t care if there are duplicates or not – it just cares about returning a single, 1-D array. So those duplicates survive throughout the whole operation, unless you call uniq on the final product.

I believe there’s an ActiveRecord way to avoid this as well: If a User has_many :collections, then @user.collections shouldn’t have any duplicates. Might be an ugly AR macro, though, with so many levels of inheritance.

Anyway, hope that helps!