Introduce NotificationSetting model

app/models/notification_setting.rb

+class NotificationSetting < ActiveRecord::Base
+  belongs_to :user
+  belongs_to :source, polymorphic: true
+
+  validates :user, presence: true
+  validates :source, presence: true
+  validates :level, presence: true
+  validates :user_id, uniqueness: { scope: [:source_type, :source_id],
+                                    message: "already exists in source",
+                                    allow_nil: true }
+  # Notification level
+  # Note: When adding an option, it MUST go on the end of the array.
+  enum level: [:disabled, :participating, :watch, :global, :mention]
+end

db/migrate/20160328112808_create_notification_settings.rb

+class CreateNotificationSettings < ActiveRecord::Migration
+  def change
+    create_table :notification_settings do |t|
+      t.integer :user_id
+      t.integer :level
+      t.integer :source_id
+      t.string :source_type
+
+      t.timestamps null: false
+    end
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160320204112) do
+ActiveRecord::Schema.define(version: 20160328112808) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -417,9 +417,9 @@ ActiveRecord::Schema.define(version: 20160320204112) do
     t.string   "state"
     t.integer  "iid"
     t.integer  "updated_by_id"
-    t.integer  "moved_to_id"
-    t.boolean  "confidential",              default: false
+    t.boolean  "confidential",  default: false
     t.datetime "deleted_at"
+    t.integer  "moved_to_id"
   end
 
   add_index "issues", ["assignee_id"], name: "index_issues_on_assignee_id", using: :btree
@@ -638,6 +638,15 @@ ActiveRecord::Schema.define(version: 20160320204112) do
   add_index "notes", ["project_id"], name: "index_notes_on_project_id", using: :btree
   add_index "notes", ["updated_at"], name: "index_notes_on_updated_at", using: :btree
 
+  create_table "notification_settings", force: :cascade do |t|
+    t.integer  "user_id"
+    t.integer  "level"
+    t.integer  "source_id"
+    t.string   "source_type"
+    t.datetime "created_at",  null: false
+    t.datetime "updated_at",  null: false
+  end
+
   create_table "oauth_access_grants", force: :cascade do |t|
     t.integer  "resource_owner_id", null: false
     t.integer  "application_id",    null: false

spec/models/notification_setting_spec.rb

+require 'rails_helper'
+
+RSpec.describe NotificationSetting, type: :model do
+  describe "Associations" do
+    it { is_expected.to belong_to(:user) }
+  end
+
+  describe "Validation" do
+    subject { NotificationSetting.new }
+
+    it { is_expected.to validate_presence_of(:user) }
+    it { is_expected.to validate_presence_of(:source) }
+    it { is_expected.to validate_presence_of(:level) }
+  end
+end