Merge branch 'fix-collection_item_serializer' into 'master'

CollectionMinSerializer

See merge request !334

app/serializers/collection_item_serializer.rb

 class CollectionItemSerializer < ActiveModel::Serializer
   cache key: 'collection_item', expires_in: 24.hours
   def collectionable
-    serializer = object.collectionable_type == "LearningObject" ?  ::LearningObjectSerializer : ::CollectionSerializer
+    serializer = object.collectionable_type == "LearningObject" ?  ::LearningObjectSerializer : ::CollectionMinSerializer
     serializer.new(object.collectionable, {scope: current_user, scope_name: :current_user}).serializable_hash
   end
   belongs_to :collectionable, polymorphic: true

app/serializers/collection_min_serializer.rb

+class CollectionMinSerializer < ActiveModel::Serializer
+  cache key: 'collection', expires_in: 4.hours
+  attributes :id, :name, :owner, :created_at, :updated_at, :description, :privacy, :score, :likes_count, :review_average, :thumbnail
+  has_many :tags
+  has_many :subjects
+  has_many :educational_stages
+end