Refactor Mentionable#notice_added_references

It now accounts for models that have changed but have already been
persisted, such as when called from an UpdateService.

Closes #1773

app/models/concerns/mentionable.rb

@@ -82,19 +82,33 @@ module Mentionable
   # If the mentionable_text field is about to change, locate any *added* references and create cross references for
   # them. Invoke from an observer's #before_save implementation.
   def notice_added_references(p = project, a = author)
-    ch = changed_attributes
-    original, mentionable_changed = "", false
-    self.class.mentionable_attrs.each do |attr|
-      if ch[attr]
-        original << ch[attr]
-        mentionable_changed = true
-      end
-    end
+    changes = detect_mentionable_changes
+
+    return if changes.empty?
 
-    # Only proceed if the saved changes actually include a chance to an attr_mentionable field.
-    return unless mentionable_changed
+    original_text = changes.collect { |_, vals| vals.first }.join(' ')
 
-    preexisting = references(p, self.author, original)
+    preexisting = references(p, self.author, original_text)
     create_cross_references!(p, a, preexisting)
   end
+
+  private
+
+  # Returns a Hash of changed mentionable fields
+  #
+  # Preference is given to the `changes` Hash, but falls back to
+  # `previous_changes` if it's empty (i.e., the changes have already been
+  # persisted).
+  #
+  # See ActiveModel::Dirty.
+  #
+  # Returns a Hash.
+  def detect_mentionable_changes
+    source = (changes.present? ? changes : previous_changes).dup
+
+    mentionable = self.class.mentionable_attrs
+
+    # Only include changed fields that are mentionable
+    source.select { |key, val| mentionable.include?(key) }
+  end
 end

spec/models/concerns/mentionable_spec.rb

@@ -28,4 +28,53 @@ describe Issue, "Mentionable" do
       issue.create_cross_references!(project, author, [commit2])
     end
   end
+
+  describe '#notice_added_references' do
+    let(:project) { create(:project) }
+    let(:issues)  { create_list(:issue, 2, project: project) }
+
+    context 'before changes are persisted' do
+      it 'ignores pre-existing references' do
+        issue = create_issue(description: issues[0].to_reference)
+
+        expect(SystemNoteService).not_to receive(:cross_reference)
+
+        issue.description = 'New description'
+        issue.notice_added_references
+      end
+
+      it 'notifies new references' do
+        issue = create_issue(description: issues[0].to_reference)
+
+        expect(SystemNoteService).to receive(:cross_reference).with(issues[1], any_args)
+
+        issue.description = issues[1].to_reference
+        issue.notice_added_references
+      end
+    end
+
+    context 'after changes are persisted' do
+      it 'ignores pre-existing references' do
+        issue = create_issue(description: issues[0].to_reference)
+
+        expect(SystemNoteService).not_to receive(:cross_reference)
+
+        issue.update_attributes(description: 'New description')
+        issue.notice_added_references
+      end
+
+      it 'notifies new references' do
+        issue = create_issue(description: issues[0].to_reference)
+
+        expect(SystemNoteService).to receive(:cross_reference).with(issues[1], any_args)
+
+        issue.update_attributes(description: issues[1].to_reference)
+        issue.notice_added_references
+      end
+    end
+
+    def create_issue(description:)
+      create(:issue, project: project, description: description)
+    end
+  end
 end