Create paricipants model

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/concerns/issuable.rb

@@ -17,6 +17,7 @@ module Issuable
     has_many :label_links, as: :target, dependent: :destroy
     has_many :labels, through: :label_links
     has_many :subscriptions, dependent: :destroy, as: :subscribable
+    # has_many :participants, dependent: :destroy, as: :target
     validates :author, presence: true
     validates :title, presence: true, length: { within: 0..255 }

app/models/participant.rb

+class Participant < ActiveRecord::Base
+  belongs_to :user
+  belongs_to :target, polymorphic: true
+
+  validates :user_id,
+            uniqueness: { scope: [:target_id, :target_type] },
+            presence: true
+end