Create BoardAssignee model and set has_one relation

app/models/board_assignee.rb

+class BoardAssignee < ActiveRecord::Base
+  belongs_to :board
+  belongs_to :assignee, class_name: 'User'
+
+  validates :board, presence: true
+  validates :assignee, presence: true
+end

ee/app/models/ee/board.rb

@@ -5,9 +5,14 @@ module EE
     prepended do
       belongs_to :group
       belongs_to :milestone
-      belongs_to :assignee, class_name: 'User'
 
       has_many :board_labels
+
+      # These can safely be changed to has_many when we support
+      # multiple assignees on the board configuration.
+      has_one :board_assignee
+      has_one :assignee, through: :board_assignee
+
       has_many :labels, through: :board_labels
 
       validates :name, presence: true

spec/models/board_assignee_spec.rb

+require 'spec_helper'
+
+describe BoardAssignee do
+  describe 'relationships' do
+    it { is_expected.to belong_to(:board) }
+    it { is_expected.to belong_to(:assignee).class_name('User') }
+  end
+
+  describe 'validations' do
+    it { is_expected.to validate_presence_of(:board) }
+    it { is_expected.to validate_presence_of(:assignee) }
+  end
+end

spec/models/board_spec.rb

@@ -4,7 +4,8 @@ describe Board do
   describe 'relationships' do
     it { is_expected.to belong_to(:project) }
     it { is_expected.to belong_to(:milestone) }
-    it { is_expected.to belong_to(:assignee).class_name('User') }
+    it { is_expected.to have_one(:board_assignee) }
+    it { is_expected.to have_one(:assignee).through(:board_assignee) }
     it { is_expected.to have_many(:board_labels) }
     it { is_expected.to have_many(:labels).through(:board_labels) }