Weird Deletion null error
===

I'm trying to delete from a join table I made and I'm getting a weird error I cant figure out.

Here are my models

    class ArticlesUser < ActiveRecord::Base
     # attr_accessible :title, :body
     belongs_to :user
     belongs_to :article
    end

    class Article < ActiveRecord::Base
     attr_accessible :title
     belongs_to :user
  
     has_many :articles_users
     has_many :likes, :through => :articles_users, :class_name => 'User', :source => :user    
    end

    class User < ActiveRecord::Base 
     has_many :articles, :order => 'id DESC'
     has_and_belongs_to_many :badges
  
     has_many :articles_users
     has_many :likes, :through => :articles_users, :class_name => 'Article', :source => :article
    end

Testing in the Rails Console you can see the error:
 
    > a = Article.find(13) 
    > a.articles_users #works fine, returns an array of users who "like" the article
    > a.articles_users.where(user_id: 3) #works as well
    > a.articles_users.where(user_id: 3).first.destroy #this is where the error is thrown!

Here is there error:
    
     ActiveRecord::StatementInvalid: Mysql2::Error: Unknown column 'articles_users.' in 'where clause': DELETE FROM `articles_users` WHERE `articles_users`.`` = NULL

Why does it seem to be completely ignoring the where hash? Please help me with this I've spent hours today trying to figure it out.

Thanks!

Edit:

The `articles_users` table has 2 columns: `article_id` and `user_id` 