add collection_items model

app/models/collection.rb

 class Collection < ActiveRecord::Base
-  has_and_belongs_to_many :learning_objects
+  has_many :collection_items
+  has_many :collection_items, as: :collectionable
+  has_many :collections, :through => :collection_items, :source => :collectionable, :source_type => 'Collection'
+  has_many :learning_objects, :through => :collection_items, :source => :collectionable, :source_type => 'LearningObject'
+
+
   belongs_to :user
   has_many :reviews, as: :reviewable
 
@@ -7,4 +12,6 @@ class Collection < ActiveRecord::Base
   has_many :downloads, as: :downloadable
   has_many :likes, as: :likeable
   has_many :shares, as: :shareable
+
+  has_many :follows, as: :followable
 end

app/models/collection_item.rb

+class CollectionItem < ActiveRecord::Base
+  belongs_to :collection
+
+  belongs_to :collectionable, polymorphic: true
+end

app/models/follow.rb

+class Follow < ActiveRecord::Base
+  belongs_to :followable, polymorphic: true
+  belongs_to :user
+end

app/models/learning_object.rb

 class LearningObject < ActiveRecord::Base
   has_and_belongs_to_many :subjects
-  has_and_belongs_to_many :collections
+
+  has_many :collection_items, as: :collectionable
+  has_many :collections, through: :collection_items
+
   has_many :complaints
   has_many :reviews, as: :reviewable
 

app/models/user.rb

@@ -18,6 +18,7 @@ class User < ActiveRecord::Base
   has_many :likes
   has_many :shares
 
+  has_many :follows
   has_many :follows, as: :followable
 
   after_create :default_role

db/migrate/20160125123516_collections_learning_objects.rb

-class CollectionsLearningObjects < ActiveRecord::Migration
-  def change
-    create_table :collections_learning_objects do |t|
-      t.integer :collection_id
-      t.integer :learning_object_id
-
-      t.timestamps null: false
-    end
-  end
-end

db/migrate/20160128125336_create_collection_items.rb

+class CreateCollectionItems < ActiveRecord::Migration
+  def change
+    create_table :collection_items do |t|
+      t.references :collectionable, polymorphic: true, index: {name: "index_citems_on_ctype_and_cid"}
+
+      t.references :collection, index: true
+
+      t.integer :order
+
+      t.timestamps null: false
+    end
+  end
+end

test/fixtures/collection_items.yml

+# Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
+
+# This model initially had no columns defined.  If you add columns to the
+# model remove the '{}' from the fixture names and add the columns immediately
+# below each fixture, per the syntax in the comments below
+#
+one: {}
+# column: value
+#
+two: {}
+#  column: value

test/models/collection_item_test.rb

+require 'test_helper'
+
+class CollectionItemTest < ActiveSupport::TestCase
+  # test "the truth" do
+  #   assert true
+  # end
+end