created item groups and link banners and their associations and fixed bugs with join tables

back-office/app/models/link_banner.rb

@@ -0,0 +1,16 @@
+class LinkBanner < ActiveRecord::Base
+	
+	belongs_to :category
+	belongs_to :section
+	belongs_to :sub_category 
+	has_many :items
+
+	validates_presence_of :beginning, :ending, :image_url
+	validate :validate_beginning_before_ending
+
+   def validate_beginning_before_ending
+    if beginning && ending
+      errors.add(:ending, "End date must come after beginning date") if ending <= beginning
+    end
+  end
+end