diff --git a/app/models/issue.rb b/app/models/issue.rb
index ab7473d23c3..d8826b65fcc 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -97,6 +97,7 @@ class Issue < ActiveRecord::Base
     end
   end
 
+  # `from` argument can be a Namespace or Project.
   def to_reference(from = nil, full: false)
     reference = "#{self.class.reference_prefix}#{iid}"
 
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 67d4c70dc3a..082adcafcc8 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -179,6 +179,7 @@ class MergeRequest < ActiveRecord::Base
     work_in_progress?(title) ? title : "WIP: #{title}"
   end
 
+  # `from` argument can be a Namespace or Project.
   def to_reference(from = nil, full: false)
     reference = "#{self.class.reference_prefix}#{iid}"
 
diff --git a/app/models/project.rb b/app/models/project.rb
index 23b98916b2e..4396ea9ef36 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -591,6 +591,7 @@ class Project < ActiveRecord::Base
     end
   end
 
+  # `from` argument can be a Namespace or Project.
   def to_reference(from = nil, full: false)
     if full || cross_namespace_reference?(from)
       path_with_namespace