Add a .for_content_id method to PreviewContent

This matches the approach we’ve agreed to take to limit
the number of service objects.

lib/engines/content_block_manager/app/models/content_block_manager/preview_content.rb

@@ -1,4 +1,24 @@
 module ContentBlockManager
   class PreviewContent < Data.define(:title, :html, :instances_count)
+    class << self
+      def for_content_id(content_id:, content_block_edition:, base_path: nil)
+        content_item = Services.publishing_api.get_content(content_id).parsed_content
+        metadata = Services.publishing_api.get_host_content_item_for_content_id(
+          content_block_edition.document.content_id,
+          content_id,
+        ).parsed_content
+        html = ContentBlockManager::GeneratePreviewHtml.new(
+          content_id:,
+          content_block_edition:,
+          base_path: base_path || content_item["base_path"],
+        ).call
+
+        ContentBlockManager::PreviewContent.new(
+          title: content_item["title"],
+          html:,
+          instances_count: metadata["instances"],
+        )
+      end
+    end
   end
 end

lib/engines/content_block_manager/test/unit/app/models/preview_content_test.rb

@@ -13,4 +13,62 @@ class ContentBlockManager::PreviewContentTest < ActiveSupport::TestCase
     assert_equal preview_content.html, html
     assert_equal preview_content.instances_count, instances_count
   end
+
+  describe ".for_content_id" do
+    let(:host_content_id) { SecureRandom.uuid }
+    let(:preview_content_id) { SecureRandom.uuid }
+    let(:host_title) { "Test" }
+    let(:host_base_path) { "/test" }
+    let(:document) do
+      build(:content_block_document, :email_address, content_id: preview_content_id)
+    end
+    let(:block_to_preview) do
+      build(:content_block_edition, :email_address, document:, details: { "email_address" => "[email protected]" }, id: 1)
+    end
+    let(:metadata_response) do
+      stub(:response, parsed_content: { "instances" => 2 })
+    end
+    let(:preview_response) { stub(:preview_response, call: html) }
+    let(:html) { "SOME_HTML" }
+
+    setup do
+      stub_publishing_api_has_item(content_id: host_content_id, title: host_title, base_path: host_base_path)
+      Services.publishing_api.expects(:get_host_content_item_for_content_id)
+              .with(block_to_preview.document.content_id, host_content_id)
+              .returns(metadata_response)
+    end
+
+    it "returns the title of host document" do
+      ContentBlockManager::GeneratePreviewHtml.expects(:new)
+                                              .with(content_id: host_content_id,
+                                                    content_block_edition: block_to_preview,
+                                                    base_path: host_base_path)
+                                              .returns(preview_response)
+
+      preview_content = ContentBlockManager::PreviewContent.for_content_id(
+        content_id: host_content_id,
+        content_block_edition: block_to_preview,
+      )
+
+      assert_equal host_title, preview_content.title
+      assert_equal 2, preview_content.instances_count
+      assert_equal html, preview_content.html
+    end
+
+    it "allows a base_path to be provided" do
+      base_path = "/something/different"
+
+      ContentBlockManager::GeneratePreviewHtml.expects(:new)
+                                              .with(content_id: host_content_id,
+                                                    content_block_edition: block_to_preview,
+                                                    base_path:)
+                                              .returns(preview_response)
+
+      ContentBlockManager::PreviewContent.for_content_id(
+        content_id: host_content_id,
+        content_block_edition: block_to_preview,
+        base_path:,
+      )
+    end
+  end
 end