Improve merge request diff creation and update tests

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/merge_request.rb

@@ -299,12 +299,8 @@ class MergeRequest < ActiveRecord::Base
   def reload_diff
     return unless open?
 
-    merge_request_diff = merge_request_diffs.create
-
     old_diff_refs = self.diff_refs
-
-    merge_request_diff.reload_content
-
+    create_merge_request_diff
     new_diff_refs = self.diff_refs
 
     update_diff_notes_positions(

spec/models/merge_request_spec.rb

@@ -9,7 +9,7 @@ describe MergeRequest, models: true do
     it { is_expected.to belong_to(:target_project).with_foreign_key(:target_project_id).class_name('Project') }
     it { is_expected.to belong_to(:source_project).with_foreign_key(:source_project_id).class_name('Project') }
     it { is_expected.to belong_to(:merge_user).class_name("User") }
-    it { is_expected.to have_one(:merge_request_diff).dependent(:destroy) }
+    it { is_expected.to have_many(:merge_request_diffs).dependent(:destroy) }
   end
 
   describe 'modules' do
@@ -134,7 +134,7 @@ describe MergeRequest, models: true do
 
     context 'when there are MR diffs' do
       it 'delegates to the MR diffs' do
-        merge_request.merge_request_diff = MergeRequestDiff.new
+        merge_request.merge_request_diffs.build
 
         expect(merge_request.merge_request_diff).to receive(:diffs).with(options)
 
@@ -654,22 +654,26 @@ describe MergeRequest, models: true do
 
     let(:commit) { subject.project.commit(sample_commit.id) }
 
-    it "reloads the diff content" do
-      expect(subject.merge_request_diff).to receive(:reload_content)
-
+    it "does not change existing merge request diff" do
+      expect(subject.merge_request_diff).not_to receive(:reload_content)
       subject.reload_diff
     end
 
+    it "creates new merge request diff" do
+      expect { subject.reload_diff }.to change { subject.merge_request_diffs.count }.by(1)
+    end
+
     it "updates diff note positions" do
       old_diff_refs = subject.diff_refs
 
-      merge_request_diff = subject.merge_request_diff
-
       # Update merge_request_diff so that #diff_refs will return commit.diff_refs
-      allow(merge_request_diff).to receive(:reload_content) do
-        merge_request_diff.base_commit_sha = commit.parent_id
-        merge_request_diff.start_commit_sha = commit.parent_id
-        merge_request_diff.head_commit_sha = commit.sha
+      allow(subject).to receive(:create_merge_request_diff) do
+        subject.merge_request_diffs.create(
+          importing: true,
+          base_commit_sha: commit.parent_id,
+          start_commit_sha: commit.parent_id,
+          head_commit_sha: commit.sha
+        )
       end
 
       expect(Notes::DiffPositionUpdateService).to receive(:new).with(
@@ -679,8 +683,8 @@ describe MergeRequest, models: true do
         new_diff_refs: commit.diff_refs,
         paths: note.position.paths
       ).and_call_original
-      expect_any_instance_of(Notes::DiffPositionUpdateService).to receive(:execute).with(note)
 
+      expect_any_instance_of(Notes::DiffPositionUpdateService).to receive(:execute).with(note)
       expect_any_instance_of(DiffNote).to receive(:save).once
 
       subject.reload_diff