Add MergeRequest#branch_merge_base_commit method

app/models/merge_request.rb

@@ -177,8 +177,8 @@ class MergeRequest < ActiveRecord::Base
   def diff_base_commit
     if persisted?
       merge_request_diff.base_commit
-    elsif diff_start_commit && diff_head_commit
-      self.target_project.merge_base_commit(diff_start_sha, diff_head_sha)
+    else
+      branch_merge_base_commit
     end
   end
 
@@ -239,6 +239,15 @@ class MergeRequest < ActiveRecord::Base
     target_project.repository.commit(target_branch) if target_branch_ref
   end
 
+  def branch_merge_base_commit
+    start_sha = target_branch_sha
+    head_sha  = source_branch_sha
+
+    if start_sha && head_sha
+      target_project.merge_base_commit(start_sha, head_sha)
+    end
+  end
+
   def target_branch_sha
     @target_branch_sha || target_branch_head.try(:sha)
   end
@@ -247,17 +256,27 @@ class MergeRequest < ActiveRecord::Base
     @source_branch_sha || source_branch_head.try(:sha)
   end
 
+  def branch_merge_base_sha
+    branch_merge_base_commit.try(:sha)
+  end
+
   def diff_refs
     if merge_request_diff
       merge_request_diff.diff_refs
-    elsif diff_start_commit || diff_base_commit
+    else
+      start_sha = target_branch_sha
+      head_sha = source_branch_sha
+      base_sha = branch_merge_base_sha
+
+      if start_sha || base_sha
         Gitlab::Diff::DiffRefs.new(
-        base_sha:  diff_base_sha,
-        start_sha: diff_start_sha,
-        head_sha:  diff_head_sha
+          base_sha:  base_sha,
+          start_sha: start_sha,
+          head_sha:  head_sha
         )
       end
     end
+  end
 
   def validate_branches
     if target_project == source_project && target_branch == source_branch

spec/models/merge_request_spec.rb

@@ -691,4 +691,21 @@ describe MergeRequest, models: true do
       subject.reload_diff
     end
   end
+
+  describe '#branch_merge_base_commit' do
+    context 'source and target branch exist' do
+      it { expect(subject.branch_merge_base_commit.sha).to eq('ae73cb07c9eeaf35924a10f713b364d32b2dd34f') }
+      it { expect(subject.branch_merge_base_commit).to be_a(Commit) }
+    end
+
+    context 'when the target branch does not exist' do
+      before do
+        subject.project.repository.raw_repository.delete_branch(subject.target_branch)
+      end
+
+      it 'returns nil' do
+        expect(subject.branch_merge_base_commit).to be_nil
+      end
+    end
+  end
 end