Fix: MergeRequestSerializer breaks on when source_project doesn't exist anymore

app/models/merge_request.rb

@@ -788,6 +788,9 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def rebase_in_progress?
+    # The source project can be deleted
+    return false unless source_project
+
     File.exist?(rebase_dir_path) && !clean_stuck_rebase
   end
 

changelogs/unreleased-ee/1289-merge-request-serializer-breaks.yml

+---
+title: 'Fix: MergeRequestSerializer breaks on MergeRequest#rebase_dir_path when source_project
+  doesn''t exist anymore'
+merge_request:
+author:

spec/models/merge_request_spec.rb

@@ -752,20 +752,32 @@ describe MergeRequest, models: true do
   end
 
   describe '#rebase_in_progress?' do
-    it 'return true' do
+    it 'returns true' do
       allow(File).to receive(:exist?).and_return(true)
       allow(File).to receive(:new).and_return(double(:file, mtime: Time.now))
+
       expect(subject.rebase_in_progress?).to be_truthy
     end
 
-    it 'return false' do
+    it 'returns false' do
       allow(File).to receive(:exist?).with(subject.rebase_dir_path).and_return(false)
+
       expect(subject.rebase_in_progress?).to be_falsey
     end
 
-    it 'return false if temporary file exists by is expired' do
+    it 'returns false if temporary file exists by is expired' do
       allow(File).to receive(:exist?).and_return(true)
       allow(File).to receive(:new).and_return(double(:file, mtime: Time.now - 2.hours))
+
+      expect(subject.rebase_in_progress?).to be_falsey
+    end
+
+    it 'returns false if source_project is removed' do
+      allow(subject).to receive(:source_project).and_return(nil)
+      allow(File).to receive(:exist?).and_return(true)
+      allow(File).to receive(:new).and_return(double(:file, mtime: Time.now))
+
+      expect(File).not_to have_received(:exist?)
       expect(subject.rebase_in_progress?).to be_falsey
     end
   end