title: Getting "Unknown primary key for table" while the ID is there
body: <p>I've been debugging this strange problem of Rails giving me "Unknown primary key for table...", even when the table's ID is there.</p>
  
  <p>I've copied the database from one heroku app to another, on the original databse there is no problem and the new one gives me a db error.</p>
  
  <p>This is the error:</p>
  
  <pre><code>ProductsController# (ActionView::Template::Error) "Unknown primary key for table collections in model Collection."
  
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/reflection.rb:366:in `primary_key'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/reflection.rb:480:in `association_primary_key'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/association_scope.rb:58:in `block in add_constraints'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/association_scope.rb:39:in `each'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/association_scope.rb:39:in `each_with_index'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/association_scope.rb:39:in `add_constraints'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/association_scope.rb:31:in `scope'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/association.rb:98:in `association_scope'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/association.rb:87:in `scoped'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/collection_association.rb:573:in `first_or_last'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/collection_association.rb:105:in `last'
  /app/vendor/bundle/ruby/2.0.0/gems/activerecord-3.2.13/lib/active_record/associations/collection_proxy.rb:46:in `last'
  /app/app/helpers/likes_helper.rb:62:in `significant_liker'
  </code></pre>
  
  <p>The line that causes it:</p>
  
  <pre><code>product.collections.last.try :user
  </code></pre>
  
  <p>and the table:</p>
  
  <pre><code>d8apjspa441pad=&gt; \d collections
                                       Table "public.collections"
       Column     |          Type          |                        Modifiers                         
  ----------------+------------------------+----------------------------------------------------------
   id             | integer                | not null default nextval('collections_id_seq'::regclass)
   name           | character varying(255) | 
   user_id        | integer                | 
   permalink      | character varying(255) | 
   category_id    | integer                | 
   products_count | integer                | 
   is_featured    | boolean                | 
  Indexes:
      "index_lists_on_user_id_and_permalink" UNIQUE, btree (user_id, permalink)
  </code></pre>
  
  <p>Any idea why this might happen?</p>
  
  <p>Thanks!</p>