Restrict the number of permitted boards per project to one

app/models/project.rb

@@ -16,6 +16,7 @@ class Project < ActiveRecord::Base
 
   extend Gitlab::ConfigHelper
 
+  NUMBER_OF_PERMITTED_BOARDS = 1
   UNKNOWN_IMPORT_URL = 'http://unknown.git'
 
   cache_markdown_field :description, pipeline: :description
@@ -65,7 +66,7 @@ class Project < ActiveRecord::Base
   belongs_to :namespace
 
   has_one :last_event, -> {order 'events.created_at DESC'}, class_name: 'Event', foreign_key: 'project_id'
-  has_many :boards, dependent: :destroy
+  has_many :boards, before_add: :validate_board_limit, dependent: :destroy
 
   # Project services
   has_many :services
@@ -1338,4 +1339,8 @@ class Project < ActiveRecord::Base
 
     shared_projects.any?
   end
+
+  def validate_board_limit(board)
+    raise StandardError, 'Number of permitted boards exceeded' if boards.size >= NUMBER_OF_PERMITTED_BOARDS
+  end
 end

spec/models/project_spec.rb

@@ -94,6 +94,15 @@ describe Project, models: true do
         end
       end
     end
+
+    describe '#boards' do
+      it 'raises an error when attempting to add more than one board to the project' do
+        subject.boards.build
+
+        expect { subject.boards.build }.to raise_error(StandardError, 'Number of permitted boards exceeded')
+        expect(subject.boards.size).to eq 1
+      end
+    end
   end
 
   describe 'modules' do