Update and correct ERD to Class Diagrams

These diagrams are not ERDs but in fact Class Diagrams. They also needed some useful context and instructions on how to update them.

Author: chao-xian

diagrams/class-diagrams/README.md

@@ -0,0 +1,37 @@
+# Class Diagrams for GOV.UK Forms apps
+
+The mermaid files in this directory are [Class Diagrams](https://mermaid.js.org/syntax/classDiagram.html) for each of the apps we run that make up the main GOV.UK Forms system.
+
+## How to generate
+
+In each app, install the `rails-erd` gem:
+
+```ruby
+group :development do
+  gem "rails-erd", git: "https://github.com/voormedia/rails-erd.git"
+end
+```
+
+We need the latest version in the repo, as at the time of writing the most recent release does not include [Mermaid support](https://github.com/voormedia/rails-erd/commit/22cfc76098e222b7c211f0369e80580aeaa22c70).
+
+> ⚠️ Note: Do not commit the gem into the app repos.
+
+Run:
+
+```shell
+generator="mermaid" rake erd
+```
+
+This will create a file called `erd.mmd` in the root of the application.
+
+## Update the diagram
+
+1. Open the file in an editor and copy the contents (all of the `classDiagram` block).
+2. Paste the contents into the corresponding diagram file, replacing everything inside the "mermaid" block:
+````
+```mermaid
+classDiagram
+...
+```
+````
+3. Update the date near the top of the file.

diagrams/admin-erd.mmd diagrams/class-diagrams/admin.mddiagrams/admin-erd.mmd renamed to diagrams/class-diagrams/admin.md

@@ -1,3 +1,11 @@
+---
+title: GOV.UK Forms Admin app class diagram
+---
+
+# GOV.UK Forms Admin app class diagram
+## 2025-05-29
+
+```mermaid
 classDiagram
 	direction RL
 	class `CreateFormService::CreateFormEvent`
@@ -77,4 +85,5 @@ classDiagram
 	`Organisation` --> `Group`
 	`Organisation` --> `MouSignature`
 	`Group` --> `Membership`
-	`Group` --> `GroupForm`
+	`Group` --> `GroupForm`
+  ```

diagrams/api-erd.mmd diagrams/class-diagrams/api.mddiagrams/api-erd.mmd renamed to diagrams/class-diagrams/api.md

@@ -1,3 +1,11 @@
+---
+title: GOV.UK Forms API class diagram
+---
+
+# GOV.UK Forms API class diagram
+## 2025-05-29
+
+```mermaid
 classDiagram
 	direction RL
 	class `AccessToken`
@@ -86,4 +94,5 @@ classDiagram
 	`Page` --> `Condition`
 	`Form` --> `MadeLiveForm`
 	`Form` --> `Api::V2::FormDocument`
-	`Condition` .. `Form`
+	`Condition` .. `Form`
+```

diagrams/runner-erd.mmd diagrams/class-diagrams/runner.mddiagrams/runner-erd.mmd renamed to diagrams/class-diagrams/runner.md

@@ -1,3 +1,11 @@
+---
+title: GOV.UK Forms Runner app class diagram
+---
+
+# GOV.UK Forms Runner app class diagram
+## 2025-05-29
+
+```mermaid
 classDiagram
 	direction RL
 	class `Submission`
@@ -8,4 +16,5 @@ classDiagram
 	`Submission` : +string mail_status
 	`Submission` : +string mode
 	`Submission` : +string reference
-	`Submission` : +datetime sent_at
+	`Submission` : +datetime sent_at
+  ```