add subject relationship

app/models/concerns/filterable.rb

+module Filterable
+  extend ActiveSupport::Concern
+
+  included do
+    has_many :subjects, through: :subject_relations, dependent: :destroy
+    has_many :subject_relations, as: :subjectable, dependent: :destroy
+  end
+end

app/models/learning_object.rb

@@ -38,6 +38,7 @@ class LearningObject < ApplicationRecord
   include Scoreable
   include Thumbnailable
   include Taggable
+  include Filterable
   include Highlights
 
   # *current_user* create learning object

app/models/subject.rb

 class Subject < ApplicationRecord
+  has_many :subject_relations
+  validates_presence_of :name
 end

app/models/subject_relation.rb

+class SubjectRelation < ApplicationRecord
+  belongs_to :subject
+  belongs_to :subjectable, polymorphic: true
+
+  validates_presence_of :subjectable, :subject
+end

db/migrate/20161026134028_create_subject_relations.rb

+class CreateSubjectRelations < ActiveRecord::Migration[5.0]
+  def change
+    create_table :subject_relations do |t|
+      t.references :subject, foreign_key: true, index: true
+      t.references :subjectable, polymorphic: true, index: true
+      t.timestamps null: false
+    end
+  end
+end

test/fixtures/subject_relations.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/subject_relation_test.rb

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