Fixed cache_key ids

Added head and base methods to ContextCommitsDiff required by a diffable to get
cache keys and added feature spec tests for the same.

Changelog: fixed

app/models/context_commits_diff.rb

@@ -3,6 +3,7 @@
 class ContextCommitsDiff
   include ActsAsPaginatedDiff
 
+  delegate :head, :base, to: :compare
   attr_reader :merge_request
 
   def initialize(merge_request)

spec/requests/projects/merge_requests/context_commit_diffs_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'Merge Requests Context Commit Diffs' do
+  let_it_be(:sha1) { "33f3729a45c02fc67d00adb1b8bca394b0e761d9" }
+  let_it_be(:project) { create(:project, :repository) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:merge_request) { create(:merge_request, target_project: project, source_project: project) }
+  let_it_be(:merge_request_context_commit) { create(:merge_request_context_commit, merge_request: merge_request, sha: sha1) }
+
+  before do
+    project.add_maintainer(user)
+    sign_in(user)
+  end
+
+  describe 'GET diffs_batch' do
+    let(:headers) { {} }
+
+    shared_examples_for 'serializes diffs with expected arguments' do
+      it 'serializes paginated merge request diff collection' do
+        expect_next_instance_of(PaginatedDiffSerializer) do |instance|
+          expect(instance).to receive(:represent)
+            .with(an_instance_of(collection), expected_options)
+            .and_call_original
+        end
+
+        subject
+      end
+    end
+
+    def collection_arguments(pagination_data = {})
+      {
+        environment: nil,
+        merge_request: merge_request,
+        commit: nil,
+        diff_view: :inline,
+        merge_ref_head_diff: nil,
+        allow_tree_conflicts: true,
+        pagination_data: {
+          total_pages: nil
+        }.merge(pagination_data)
+      }
+    end
+
+    def go(extra_params = {})
+      params = {
+        namespace_id: project.namespace.to_param,
+        project_id: project,
+        id: merge_request.iid,
+        only_context_commits: true,
+        page: 0,
+        per_page: 20,
+        format: 'json'
+      }
+
+      get diffs_batch_namespace_project_json_merge_request_path(params.merge(extra_params)), headers: headers
+    end
+
+    context 'with caching', :use_clean_rails_memory_store_caching do
+      subject { go(page: 0, per_page: 5) }
+
+      context 'when the request has not been cached' do
+        it_behaves_like 'serializes diffs with expected arguments' do
+          let(:collection) { Gitlab::Diff::FileCollection::Compare }
+          let(:expected_options) { collection_arguments }
+        end
+      end
+
+      context 'when the request has already been cached' do
+        before do
+          go(page: 0, per_page: 5)
+        end
+
+        it 'does not serialize diffs' do
+          expect_next_instance_of(PaginatedDiffSerializer) do |instance|
+            expect(instance).not_to receive(:represent)
+          end
+
+          subject
+        end
+
+        context 'with the different user' do
+          let(:another_user) { create(:user) }
+
+          before do
+            project.add_maintainer(another_user)
+            sign_in(another_user)
+          end
+
+          it_behaves_like 'serializes diffs with expected arguments' do
+            let(:collection) { Gitlab::Diff::FileCollection::Compare }
+            let(:expected_options) { collection_arguments }
+          end
+        end
+      end
+    end
+  end
+end