Prevent PG::Error exception when check commit reference on commit

app/models/note.rb

@@ -47,7 +47,7 @@ class Note < ActiveRecord::Base
   scope :for_commit_id, ->(commit_id) { where(noteable_type: "Commit", commit_id: commit_id) }
   scope :inline, ->{ where("line_code IS NOT NULL") }
   scope :not_inline, ->{ where(line_code: [nil, '']) }
-
+  scope :system, ->{ where(system: true) }
   scope :common, ->{ where(noteable_type: ["", nil]) }
   scope :fresh, ->{ order("created_at ASC, id ASC") }
   scope :inc_author_project, ->{ includes(:project, :author) }
@@ -168,9 +168,14 @@ class Note < ActiveRecord::Base
     # Determine whether or not a cross-reference note already exists.
     def cross_reference_exists?(noteable, mentioner)
       gfm_reference = mentioner_gfm_ref(noteable, mentioner)
+      notes = if noteable.is_a?(Commit)
+                where(commit_id: noteable.id)
+              else
+                where(noteable_id: noteable.id)
+              end
 
-      where(['noteable_id = ? and system = ? and note like ?',
-             noteable.id, true, "_mentioned in #{gfm_reference}_"]).any?
+      notes.where('note like ?', "_mentioned in #{gfm_reference}_").
+        system.any?
     end
 
     def search(query)

spec/models/note_spec.rb

@@ -258,6 +258,17 @@ describe Note do
       its(:commit_id) { should == commit.id }
       its(:note) { should == "_mentioned in issue ##{issue.iid}_" }
     end
+
+    context 'commit from commit' do
+      let(:parent_commit) { commit.parents.first }
+      subject { Note.create_cross_reference_note(commit, parent_commit, author, project) }
+
+      it { should be_valid }
+      its(:noteable_type) { should == "Commit" }
+      its(:noteable_id) { should be_nil }
+      its(:commit_id) { should == commit.id }
+      its(:note) { should == "_mentioned in commit #{parent_commit.id[0...6]}_" }
+    end
   end
 
   describe '#cross_reference_exists?' do
@@ -278,6 +289,15 @@ describe Note do
     it 'detects if a mentionable has not already been mentioned' do
       Note.cross_reference_exists?(issue, commit1).should be_false
     end
+
+    context 'commit on commit' do
+      before do
+        Note.create_cross_reference_note(commit0, commit1, author, project)
+      end
+
+      it { Note.cross_reference_exists?(commit0, commit1).should be_true }
+      it { Note.cross_reference_exists?(commit1, commit0).should be_false }
+    end
   end
 
   describe '#system?' do