diff --git a/app/models/concerns/mentionable.rb b/app/models/concerns/mentionable.rb
index d640728519a..50be458bf24 100644
--- a/app/models/concerns/mentionable.rb
+++ b/app/models/concerns/mentionable.rb
@@ -67,9 +67,10 @@ module Mentionable
     return [] if text.blank?
     ext = Gitlab::ReferenceExtractor.new
     ext.analyze(text, p)
-    (ext.issues_for +
-     ext.merge_requests_for +
-     ext.commits_for).uniq - [local_reference]
+
+    (ext.issues_for(p)  +
+     ext.merge_requests_for(p) +
+     ext.commits_for(p)).uniq - [local_reference]
   end
 
   # Create a cross-reference Note for each GFM reference to another Mentionable found in +mentionable_text+.
diff --git a/app/models/external_issue.rb b/app/models/external_issue.rb
new file mode 100644
index 00000000000..50efcb32f1b
--- /dev/null
+++ b/app/models/external_issue.rb
@@ -0,0 +1,25 @@
+class ExternalIssue
+  def initialize(issue_identifier, project)
+    @issue_identifier, @project = issue_identifier, project
+  end
+
+  def to_s
+    @issue_identifier.to_s
+  end
+
+  def id
+    @issue_identifier.to_s
+  end
+
+  def iid
+    @issue_identifier.to_s
+  end
+
+  def ==(other)
+    other.is_a?(self.class) && (to_s == other.to_s)
+  end
+
+  def project
+    @project
+  end
+end
diff --git a/lib/gitlab/reference_extractor.rb b/lib/gitlab/reference_extractor.rb
index 0b9177afa4f..7e5c991a222 100644
--- a/lib/gitlab/reference_extractor.rb
+++ b/lib/gitlab/reference_extractor.rb
@@ -71,7 +71,7 @@ module Gitlab
       if entry_project.nil?
         false
       else
-        project.nil? || project.id == entry_project.id
+        project.nil? || entry_project.default_issues_tracker?
       end
     end
   end