Clear caches before updating MR diffs

app/models/merge_request.rb

@@ -53,8 +53,8 @@ class MergeRequest < ActiveRecord::Base
   serialize :merge_params, Hash # rubocop:disable Cop/ActiveRecordSerialize
 
   after_create :ensure_merge_request_diff, unless: :importing?
-  after_update :reload_diff_if_branch_changed
   after_update :clear_memoized_shas
+  after_update :reload_diff_if_branch_changed
 
   # When this attribute is true some MR validation is ignored
   # It allows us to close or modify broken merge requests

spec/models/merge_request_spec.rb

@@ -601,30 +601,30 @@ describe MergeRequest do
   end
 
   describe '#can_remove_source_branch?' do
-    let(:user) { create(:user) }
-    let(:user2) { create(:user) }
+    set(:user) { create(:user) }
+    set(:merge_request) { create(:merge_request, :simple) }
 
-    before do
-      subject.source_project.team << [user, :master]
+    subject { merge_request }
 
-      subject.source_branch = "feature"
-      subject.target_branch = "master"
-      subject.save!
+    before do
+      subject.source_project.add_master(user)
     end
 
     it "can't be removed when its a protected branch" do
       allow(ProtectedBranch).to receive(:protected?).and_return(true)
+
       expect(subject.can_remove_source_branch?(user)).to be_falsey
     end
 
     it "can't remove a root ref" do
-      subject.source_branch = "master"
-      subject.target_branch = "feature"
+      subject.update(source_branch: 'master', target_branch: 'feature')
 
       expect(subject.can_remove_source_branch?(user)).to be_falsey
     end
 
     it "is unable to remove the source branch for a project the user cannot push to" do
+      user2 = create(:user)
+
       expect(subject.can_remove_source_branch?(user2)).to be_falsey
     end
 
@@ -635,6 +635,7 @@ describe MergeRequest do
     end
 
     it "cannot be removed if the last commit is not also the head of the source branch" do
+      subject.clear_memoized_shas
       subject.source_branch = "lfs"
 
       expect(subject.can_remove_source_branch?(user)).to be_falsey
@@ -1405,6 +1406,16 @@ describe MergeRequest do
 
       subject.reload_diff
     end
+
+    context 'when using the after_update hook to update' do
+      context 'when the branches are updated' do
+        it 'uses the new heads to generate the diff' do
+          expect { subject.update!(source_branch: subject.target_branch, target_branch: subject.source_branch) }
+            .to change { subject.merge_request_diff.start_commit_sha }
+            .and change { subject.merge_request_diff.head_commit_sha }
+        end
+      end
+    end
   end
 
   describe '#update_diff_discussion_positions' do

spec/services/merge_requests/merge_service_spec.rb

 require 'spec_helper'
 
 describe MergeRequests::MergeService do
-  let(:user) { create(:user) }
-  let(:user2) { create(:user) }
+  set(:user) { create(:user) }
+  set(:user2) { create(:user) }
   let(:merge_request) { create(:merge_request, :simple, author: user2, assignee: user2) }
   let(:project) { merge_request.project }
 
   before do
-    project.team << [user, :master]
-    project.team << [user2, :developer]
+    project.add_master(user)
+    project.add_developer(user2)
   end
 
   describe '#execute' do