Add Branch model

This adds a `Branch` model and a representation of a data store's
default branch, allowing us to remove the more specific environment
variable/app configuration.

Branches are annoying because they are undocumented and don't formally
form part of the Discovery Engine API. Every data store has exactly one
branch (the default one), and you don't interact with it in any way
_other than_ when you add/update/remove documents on the datastore.

app/models/branch.rb

@@ -0,0 +1,21 @@
+# Represents a branch on a Discovery Engine data store.
+#
+# Currently, every data store on Discovery Engine has exactly *one* branch (the default branch), and
+# we are not able to make any changes to that resource. However, we still need to model it here
+# because documents are children of the branch, not the datastore itself.
+#
+# see https://cloud.google.com/ruby/docs/reference/google-cloud-discovery_engine-v1/latest/Google-Cloud-DiscoveryEngine-V1-DocumentService-Client
+# (there is no documentation specific to branches)
+Branch = Data.define(:remote_resource_id) do
+  include DiscoveryEngineNameable
+
+  # The default branch automatically available on a data store
+  def self.default
+    new("default_branch")
+  end
+
+  def parent
+    # We only use a single data store in our architecture, so we can hardcode it here.
+    DataStore.default
+  end
+end

app/services/discovery_engine/sync/operation.rb

@@ -60,7 +60,7 @@ def version_cache
     end
 
     def document_name
-      "#{Rails.configuration.discovery_engine_datastore_branch}/documents/#{content_id}"
+      [Branch.default.name, "documents", content_id].join("/")
     end
 
     def log(level, message)

config/application.rb

@@ -19,7 +19,6 @@ class Application < Rails::Application
     # Google Discovery Engine configuration
     config.discovery_engine_default_collection_name = ENV.fetch("DISCOVERY_ENGINE_DEFAULT_COLLECTION_NAME")
     config.discovery_engine_serving_config = ENV.fetch("DISCOVERY_ENGINE_SERVING_CONFIG")
-    config.discovery_engine_datastore_branch = ENV.fetch("DISCOVERY_ENGINE_DATASTORE_BRANCH")
     config.google_cloud_project_id = ENV.fetch("GOOGLE_CLOUD_PROJECT_ID")
 
     # Document sync configuration

spec/models/branch_spec.rb

@@ -0,0 +1,15 @@
+RSpec.describe Branch do
+  subject(:branch) { described_class.new("my-branch") }
+
+  describe ".default" do
+    it "returns the default branch" do
+      expect(described_class.default).to eq(described_class.new("default_branch"))
+    end
+  end
+
+  describe "#name" do
+    it "returns the fully qualified name of the branch" do
+      expect(subject.name).to eq("[collection]/dataStores/govuk_content/branches/my-branch")
+    end
+  end
+end

spec/services/discovery_engine/sync/delete_spec.rb

@@ -14,7 +14,7 @@
 
     it "deletes the document" do
       expect(client).to have_received(:delete_document)
-        .with(name: "branch/documents/some_content_id")
+        .with(name: "#{Branch.default.name}/documents/some_content_id")
     end
 
     it_behaves_like "a successful sync operation", "delete"

spec/services/discovery_engine/sync/put_spec.rb

@@ -26,7 +26,7 @@
       expect(client).to have_received(:update_document).with(
         document: {
           id: "some_content_id",
-          name: "branch/documents/some_content_id",
+          name: "#{Branch.default.name}/documents/some_content_id",
           json_data: "{\"foo\":\"bar\",\"payload_version\":\"1\"}",
           content: {
             mime_type: "text/html",

spec/services/discovery_engine/sync/shared_examples.rb

@@ -8,7 +8,6 @@
   before do
     allow(Kernel).to receive(:sleep).and_return(nil)
     allow(Rails).to receive(:logger).and_return(logger)
-    allow(Rails.configuration).to receive(:discovery_engine_datastore_branch).and_return("branch")
     allow(GovukError).to receive(:notify)
 
     allow(Coordination::DocumentLock).to receive(:new).with("some_content_id").and_return(lock)