Allow batch_diffs to filter returned diffs by file path

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -20,7 +20,10 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
   end
 
   def diffs_batch
-    diffs = @compare.diffs_in_batch(params[:page], params[:per_page], diff_options: diff_options)
+    diff_options_hash = diff_options
+    diff_options_hash[:paths] = params[:paths] if params[:paths]
+
+    diffs = @compare.diffs_in_batch(params[:page], params[:per_page], diff_options: diff_options_hash)
     positions = @merge_request.note_positions_for_paths(diffs.diff_file_paths, current_user)
     environment = @merge_request.environments_for(current_user, latest: true).last
 

app/models/merge_request_diff_file.rb

@@ -8,6 +8,10 @@ class MergeRequestDiffFile < ApplicationRecord
   belongs_to :merge_request_diff, inverse_of: :merge_request_diff_files
   alias_attribute :index, :relative_order
 
+  scope :by_paths, ->(paths) do
+    where("new_path in (?) OR old_path in (?)", paths, paths)
+  end
+
   def utf8_diff
     return '' if diff.blank?
 

app/serializers/paginated_diff_entity.rb

@@ -11,8 +11,13 @@ class PaginatedDiffEntity < Grape::Entity
   expose :diff_files do |diffs, options|
     submodule_links = Gitlab::SubmoduleLinks.new(merge_request.project.repository)
 
-    DiffFileEntity.represent(diffs.diff_files,
-      options.merge(submodule_links: submodule_links, code_navigation_path: code_navigation_path(diffs)))
+    DiffFileEntity.represent(
+      diffs.diff_files,
+      options.merge(
+        submodule_links: submodule_links,
+        code_navigation_path: code_navigation_path(diffs)
+      )
+    )
   end
 
   expose :pagination do

lib/gitlab/diff/file_collection/merge_request_diff_batch.rb

@@ -18,10 +18,8 @@ module Gitlab
         def initialize(merge_request_diff, batch_page, batch_size, diff_options:)
           super(merge_request_diff, diff_options: diff_options)
 
-          batch_page ||= DEFAULT_BATCH_PAGE
-          batch_size ||= DEFAULT_BATCH_SIZE
+          @paginated_collection = load_paginated_collection(batch_page, batch_size, diff_options)
 
-          @paginated_collection = relation.page(batch_page).per(batch_size)
           @pagination_data = {
             current_page: @paginated_collection.current_page,
             next_page: @paginated_collection.next_page,
@@ -63,6 +61,18 @@ module Gitlab
         def relation
           @merge_request_diff.merge_request_diff_files
         end
+
+        def load_paginated_collection(batch_page, batch_size, diff_options)
+          batch_page ||= DEFAULT_BATCH_PAGE
+          batch_size ||= DEFAULT_BATCH_SIZE
+
+          paths = diff_options&.fetch(:paths, nil)
+
+          paginated_collection = relation.page(batch_page).per(batch_size)
+          paginated_collection = paginated_collection.by_paths(paths) if paths
+
+          paginated_collection
+        end
       end
     end
   end

spec/controllers/projects/merge_requests/diffs_controller_spec.rb

@@ -454,6 +454,31 @@ RSpec.describe Projects::MergeRequests::DiffsController do
       it_behaves_like 'successful request'
     end
 
+    context 'with paths param' do
+      let(:example_file_path) { "README" }
+      let(:file_path_option) { { paths: [example_file_path] } }
+
+      subject do
+        go(file_path_option)
+      end
+
+      it_behaves_like 'serializes diffs with expected arguments' do
+        let(:collection) { Gitlab::Diff::FileCollection::MergeRequestDiffBatch }
+        let(:expected_options) do
+          collection_arguments(current_page: 1, total_pages: 1)
+        end
+      end
+
+      it_behaves_like 'successful request'
+
+      it 'filters down the response to the expected file path' do
+        subject
+
+        expect(json_response["diff_files"].size).to eq(1)
+        expect(json_response["diff_files"].first["file_path"]).to eq(example_file_path)
+      end
+    end
+
     context 'with default params' do
       subject { go }