Imrpove participants model and add some basic specs

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

app/models/participant.rb

@@ -2,6 +2,7 @@ class Participant < ActiveRecord::Base
   belongs_to :user
   belongs_to :target, polymorphic: true
+  validates :target, presence: true
   validates :user_id,
             uniqueness: { scope: [:target_id, :target_type] },
             presence: true

app/models/user.rb

@@ -132,6 +132,7 @@ class User < ActiveRecord::Base
   has_many :assigned_issues,          dependent: :destroy, foreign_key: :assignee_id, class_name: "Issue"
   has_many :assigned_merge_requests,  dependent: :destroy, foreign_key: :assignee_id, class_name: "MergeRequest"
   has_many :oauth_applications, class_name: 'Doorkeeper::Application', as: :owner, dependent: :destroy
+  has_many :participants,             dependent: :destroy
   #

spec/factories/participants.rb

+# Read about factories at https://github.com/thoughtbot/factory_girl
+
+FactoryGirl.define do
+  factory :participant do
+    target factory: issue
+    user
+  end
+end

spec/models/participant_spec.rb

+require 'spec_helper'
+
+describe Participant do
+  describe 'associations' do
+    it { is_expected.to belong_to(:user) }
+    it { is_expected.to belong_to(:target) }
+  end
+end