More descriptive PR message when multiple dependencies are fixed in a security update

common/lib/dependabot/pull_request_creator/message_builder.rb

@@ -192,14 +192,22 @@ def requirement_commit_message_intro
       end
 
       # rubocop:disable Metrics/PerceivedComplexity
+      # rubocop:disable Metrics/CyclomaticComplexity
+      # rubocop:disable Metrics/AbcSize
       def version_commit_message_intro
+        dependency = dependencies.first
+
         return multidependency_property_intro if dependencies.count > 1 && updating_a_property?
 
         return dependency_set_intro if dependencies.count > 1 && updating_a_dependency_set?
 
+        return transitive_multidependency_intro if dependencies.count > 1 &&
+                                                   dependencies.any?(&:top_level?) &&
+                                                   dependencies.any? { |dep| !dep.top_level? } &&
+                                                   dependencies.none?(&:removed?)
+
         return multidependency_intro if dependencies.count > 1
 
-        dependency = dependencies.first
         msg = "Bumps #{dependency_links.first} " \
               "#{from_version_msg(previous_version(dependency))}" \
               "to #{new_version(dependency)}."
@@ -216,6 +224,8 @@ def version_commit_message_intro
       end
 
       # rubocop:enable Metrics/PerceivedComplexity
+      # rubocop:enable Metrics/CyclomaticComplexity
+      # rubocop:enable Metrics/AbcSize
 
       def multidependency_property_intro
         dependency = dependencies.first
@@ -239,6 +249,23 @@ def multidependency_intro
           "dependencies needed to be updated together."
       end
 
+      def transitive_multidependency_intro
+        dependency = dependencies.first
+
+        msg = "Bumps #{dependency_links[0]} to #{new_version(dependency)}"
+
+        msg += if dependencies.count > 2
+                 " and updates ancestor dependencies #{dependency_links[0..-2].join(', ')} " \
+                   "and #{dependency_links[-1]}. "
+               else
+                 " and updates ancestor dependency #{dependency_links[1]}. "
+               end
+
+        msg += "These dependencies need to be updated together.\n"
+
+        msg
+      end
+
       def from_version_msg(previous_version)
         return "" unless previous_version
 

common/spec/dependabot/pull_request_creator/message_builder_spec.rb

@@ -1242,6 +1242,71 @@ def commits_details(base:, head:)
         end
       end
 
+      context "and transitive security vulnerabilities fixed" do
+        let(:dependencies) { [transitive_dependency, dependency] }
+        let(:transitive_dependency) do
+          Dependabot::Dependency.new(
+            name: "statesman",
+            version: "1.6.0",
+            previous_version: "1.5.0",
+            package_manager: "dummy",
+            requirements: [],
+            previous_requirements: []
+          )
+        end
+
+        before do
+          statesman_repo_url =
+            "https://api.github.com/repos/gocardless/statesman"
+          stub_request(:get, statesman_repo_url).
+            to_return(status: 200,
+                      body: fixture("github", "statesman_repo.json"),
+                      headers: json_header)
+          stub_request(:get, "#{statesman_repo_url}/contents/").
+            to_return(status: 200,
+                      body: fixture("github", "statesman_files.json"),
+                      headers: json_header)
+          stub_request(:get, "#{statesman_repo_url}/releases?per_page=100").
+            to_return(status: 200,
+                      body: fixture("github", "business_releases.json"),
+                      headers: json_header)
+          stub_request(:get, "https://api.github.com/repos/gocardless/" \
+                             "statesman/contents/CHANGELOG.md?ref=master").
+            to_return(status: 200,
+                      body: fixture("github", "changelog_contents.json"),
+                      headers: json_header)
+          stub_request(:get, "https://rubygems.org/api/v1/gems/statesman.json").
+            to_return(
+              status: 200,
+              body: fixture("ruby", "rubygems_response_statesman.json")
+            )
+
+          service_pack_url =
+            "https://github.com/gocardless/statesman.git/info/refs" \
+            "?service=git-upload-pack"
+          stub_request(:get, service_pack_url).
+            to_return(
+              status: 200,
+              body: fixture("git", "upload_packs", "no_tags"),
+              headers: {
+                "content-type" => "application/x-git-upload-pack-advertisement"
+              }
+            )
+        end
+
+        it "includes details of both dependencies" do
+          expect(pr_message).
+            to start_with(
+              "Bumps [statesman](https://github.com/gocardless/statesman) to 1.6.0 " \
+              "and updates ancestor dependency [business](https://github.com/gocardless/business). " \
+              "These dependencies need to be updated together.\n\n" \
+              "Updates `statesman` from 1.5.0 to 1.6.0\n" \
+              "<details>\n" \
+              "<summary>Release notes</summary>\n"
+            )
+        end
+      end
+
       context "and an upgrade guide that can be pulled in" do
         let(:dependency) do
           Dependabot::Dependency.new(