how to handle "stacking order" of a collection

[MikeShank]

I suppose this may be more of a database question, but what is the best way to handle this scenario.
I have the following tables: articles, collections, articles_collections
a collection contains many articles but I want to be able to order them. Would I simply add a "stacking_order" column to the articles table?
Although, an article can belong to many collections and each would have different stacking orders.
Any suggestions?

[Luke Redpath]

It sounds like you want a join model between your Article and Collection. Without knowing more about your domain its hard to put a name to it, but lets say that model is called Foo. You'd have articles, collections and foos tables. The foos table will have an article_id, a collection_id and a stacking_order column. Your models would look a bit like this:

class Article < ActiveRecord::Base
  has_many :foos, :order => 'stacking_order ASC'
  has_many :collections, :through => :foo
end

class Collection < ActiveRecord::Base
  has_many :foos, :order => 'stacking_order'
  has_many :articles, :through => :foo
end

class Foo < ActiveRecord::Base
  belongs_to :article
  belongs_to :collection
end

Not tested, but hope that helps.

[MikeShank]

Hi Luke,
Thanks for your help, my join table would be articles_collections. Why do you use has_many through instead of has_and_belongs_to_many?

[Luke Redpath]

Because that way you can store extra information about the relationship, such as the stacking order. Take a look at this article:

http://blog.hasmanythrough.com/artic...many-dance-off

[Bob Carologees]

might be a good chance to use polymorphic associations

[MikeShank]

Hi,
I changed my has_and_belongs_to_many to use has_many through and now my tests are throwing mysql errors like "Cannot add or update a child row: a foreign key constraint fails:"
Any idea on what I'll need to do to fix?

When setting foreign keys what should set the "on delete" and "on update" handlers to CASCADE, SET_NULL, NO ACTION, RESTRICT

[Luke Redpath]

Personally, I wouldn't bother with foreign key constraints in your database unless it has to be accessed by several different apps. Use your Rails models to set your constraints. Database constraints just make things a pain in the **** to test.

[MikeShank]

Ok I'm totally confused, I removed the foreign keys and now i get "ActiveRecord::HasManyThroughAssociationNotFoundError: Could not find the association :compositions"

Here's what I had working with the has_and_belongs_to_many:

   class Collection < ActiveRecord::Base
       has_and_belongs_to_many :articles
    belongs_to :author
    
    def add(article)
        articles << article
    end

   class Author < ActiveRecord::Base
       has_many :articles
   
   class Article < ActiveRecord::Base
       belongs_to :author
       has_and_belongs_to_many :collections 

Which i have changed to :

    class Collection < ActiveRecord::Base
        has_many :articles, :through => :collection_items
     belongs_to :author
     
     def add(article)
#SHOULD I BE REFERRING TO collection_items instead of articles?
         articles << article
     end
 
    class Author < ActiveRecord::Base
    has_many :articles
    has_many :collections
    
    class Article < ActiveRecord::Base
        belongs_to :author
        has_many :collections, :through => :collection_items

class CollectionItem < ActiveRecord::Base
    belongs_to :article
    belongs_to :collection 

[Luke Redpath]

Where is the compositions coming from?

[MikeShank]

sorry about that I cut and paste from another test.
Could not find the association :collection_items

[MikeShank]

No ideas, this is driving me insane.
I mean should what I'm doing here work?
Would a collection have a reference to an articles array? or since I am going thru collection_items would I have to access something like my_collection.collection_items.articles?

I have to say I thought habtm was confusing, but this tops it. I don't know if i'm being particularly dense, but i appreciate your help so far.

[MikeShank]

Ok, lets start over.

Here are my models:

  class Article < ActiveRecord::Base
      belongs_to :author
      has_many :collections, :through => :collection_items
  end
  
  class Author < ActiveRecord::Base
      has_many :articles
      has_many :collections
  end
  
  class Collection < ActiveRecord::Base
          has_many :articles, :through => :collection_items
          belongs_to :author
          
          def add(article)
              articles << article
          end
          
          def remove(article)
              if articles.detect{|i| i == article}
                  articles.delete(article)
              end
          end
  end
  
  class CollectionItem < ActiveRecord::Base
      belongs_to :article
      belongs_to :collection
  end 

Here are my fixtures:

collection_items.yml

  item1:
    article_id: 1
    collection_id: 1
    stacking_order: 1
  item2:
    article_id: 2
    collection_id: 1
    stacking_order: 2
  item3:
    article_id: 1
    collection_id: 2
    stacking_order: 1 

articles.yml

  article1:
    id: 1
    author_id: 1
    keywords: web development
    body: This is an article about web design and development with Ruby
  
  article2:
    id: 2
    author_id: 2
    keywords: something
    body: This is an article about something else 

collections.yml

  collection1:
    id: 1
    author_id: 1
    name: newsletter
  collection2:
    id: 2
    author_id: 1
    name: webpage 

And here's a test that fails...

    fixtures :collections, :articles, :collection_items 

      def test_how_many_articles
          result = Collection.find(1)
          assert_equal 2, result.articles.length
      end 

with this error:

  test_how_many_articles(TestCollection):
  ActiveRecord::HasManyThroughAssociationNotFoundError: Could not find the association :collection_items in model Article
      /usr/lib/ruby/gems/1.8/gems/activerecord-1.14.0/lib/active_record/reflection.rb:169:in `check_validity!'
      /usr/lib/ruby/gems/1.8/gems/activerecord-1.14.0/lib/active_record/associations/has_many_through_association.rb:6:in `initialize'
      /usr/lib/ruby/gems/1.8/gems/activerecord-1.14.0/lib/active_record/associations.rb:865:in `new'
      /usr/lib/ruby/gems/1.8/gems/activerecord-1.14.0/lib/active_record/associations.rb:865:in `articles'
      /usr/lib/ruby/gems/1.8/gems/activerecord-1.14.0/lib/active_record/associations.rb:860:in `articles'
      ./test/unit/test_collection.rb:22:in `test_how_many_articles' 

What am I missing here? Am I not understanding has_many :through or is this a bug?

[MikeShank]

My god that was painful!

Finally found the problem.

found it here: wiki.rubyonrails.org

It should be made clear that the models for using through need to include an association for the through join, ie:

  class Product < ActiveRecord::Base
    has_many :catalogue_items
    has_many :catalogues, :through => :catalogue_items
  end 

If you don’t include the join association you will get an HasManyThroughAssociationNotFoundError error stating: Could not find the association :catalogue_items in model Vendor
I might be an idiot, and it might say this somewhere in the docs, but I struggled with this for a while, and couldn’t find anything on the subject.

Thanks again Luke for your help.

[MikeShank]

Ok, I'm back with a new issue.
It seems that trying to add an article to a collection:

my_collection.articles << new_article
my_collection.save 

does not work. However, it did with habtm.
Any ideas on why?

[Luke Redpath]

Hi Mike, unfortunately you cannot do that as your has_many through relationships do not act as proxy collections.

Article here:
http://blog.hasmanythrough.com/artic...xy-collections

Try this instead:

my_collection.collection_items.create(:article => new_article)

[MikeShank]

Hi Luke,
Thanks, I actually stumbled on that late last nite.
from that article:

 pat.skills.size             # => 4
 pat.skills(true).size       # => 5 

what does (true) do?

Also, to remove I assumed would be like create:

 collection_items.remove(item) 

apparently not:

 NoMethodError: undefined method `remove' for CollectionItem:Class 

[MikeShank]

Here is what I've come up with for removing:

items = collection_items.find(:all, :conditions => "article_id = #{article.id}")
items.each {|i| i.destroy } 

[MiiJaySung]

Here is what I've come up with for removing:

items = collection_items.find(:all, :conditions => "article_id = #{article.id}")
items.each {|i| i.destroy } 

If you look about, on your associations you can use :dependant => :destroy to do that for you, there is a :delete_all option, that saves some processing overhead if there is no need for any callbacks for model destruction. ....

http://api.rubyonrails.com/classes/A...ssMethods.html

explains it all in more detail

[MikeShank]

Thanks MiiJaySung, I did finally come to find that out.