Add cross project references support for label model

app/models/label.rb

@@ -70,16 +70,20 @@ class Label < ActiveRecord::Base
   #
   #   Label.first.to_reference                # => "~1"
   #   Label.first.to_reference(format: :name) # => "~\"bug\""
+  #   Label.first.to_reference(project)       # => "gitlab-org/gitlab-ce~1"
   #
   # Returns a String
-  def to_reference(_from_project = nil, format: :id)
-    if format == :name && !name.include?('"')
-      %(#{self.class.reference_prefix}"#{name}")
+  def to_reference(from_project = nil, format: :id)
+    reference = label_format_reference(format)
+
+    if cross_project_reference?(from_project)
+      project.to_reference + reference
     else
-      "#{self.class.reference_prefix}#{id}"
+      reference
     end
   end
 
+
   def open_issues_count
     issues.opened.count
   end
@@ -95,4 +99,16 @@ class Label < ActiveRecord::Base
   def template?
     template
   end
+
+  private
+
+  def label_format_reference(format = :id)
+    raise StandardError, 'Unknown format' unless [:id, :name].include?(format)
+
+    if format == :name && !name.include?('"')
+      %(#{self.class.reference_prefix}"#{name}")
+    else
+      "#{self.class.reference_prefix}#{id}"
+    end
+  end
 end

spec/models/label_spec.rb

@@ -59,7 +59,6 @@ describe Label, models: true do
     context 'using id' do
       it 'returns a String reference to the object' do
         expect(label.to_reference).to eq "~#{label.id}"
-        expect(label.to_reference(double('project'))).to eq "~#{label.id}"
       end
     end
 
@@ -73,5 +72,30 @@ describe Label, models: true do
         expect(label.to_reference(format: :name)).to eq "~#{label.id}"
       end
     end
+
+    context 'using invalid format' do
+      it 'raises error' do
+        expect { label.to_reference(format: :invalid) }
+          .to raise_error StandardError, /Unknown format/
+      end
+    end
+
+    context 'cross project reference' do
+      let(:project) { create(:project) }
+
+      context 'using name' do
+        it 'returns cross reference with label name' do
+          expect(label.to_reference(project, format: :name))
+            .to eq %Q(#{label.project.to_reference}~"#{label.name}")
+        end
+      end
+
+      context 'using id' do
+        it 'returns cross reference with label id' do
+          expect(label.to_reference(project, format: :id))
+            .to eq %Q(#{label.project.to_reference}~#{label.id})
+        end
+      end
+    end
   end
 end