doc: guide for custom auth (#395)

Author: ymc9

docs/guides/authentication/custom.md

@@ -0,0 +1,55 @@
+---
+description: Integrating with a custom authentication system.
+sidebar_position: 100
+sidebar_label: Custom Authentication
+---
+
+# Integrating With a Custom Authentication System
+
+You may be using an authentication provider that's not mentioned in the guides. Or you may have a custom-implemented one. Integrating ZenStack with any authentication system is pretty straightforward. This guide will provide the general steps to follow.
+
+## Determining what's needed for access control
+
+The bridge that connects authentication and authorization is the `auth()` function that represents the authenticated current user. Based on your requirements, you should determine what fields are needed from it. The `auth()` object must at least contain an id field that uniquely identifies the current user. If you do RBAC, you'll very likely need an `auth().role` field available. Or even an `auth().permissions` field for fine-grained control.
+
+The `auth()` call needs to be resolved to a "type" in ZModel. If you store user data in your database, you may already have a "User" model that carries all the fields you need to access.
+
+```zmodel
+model User {
+  id          String @id
+  role        String
+  permissions String[]
+  ...
+}
+```
+
+ZenStack picks up the model named "User" automatically to resolve `auth()` unless another model or type is specifically appointed (by using the `@@auth` attribute). For example, if you're not storing user data locally, you can define a "type" to resolve `auth()`. This way, you can provide typing without being backed by a database table.
+
+```zmodel
+type Auth {
+  id          String @id
+  role        String
+  permissions String[]
+  @@auth
+}
+```
+
+Just remember that any thing that you access from `auth().` must be resolved.
+
+## Fetching the current user along with the additional information
+
+At runtime in your backend code, you need to provide a value for the `auth()` call to ZenStack, so it can use it to evaluate access policies. How this is done is solely dependent on your authentication mechanism. Here are some examples:
+
+1. If you use JWT tokens, you can issue tokens with user id and other fields embedded, then validate and extract them from the request.
+2. If you use a dedicated authentication service, you can call it to get the current user's information.
+
+You must ensure that whatever approach you use, the user information you get can be trusted and free of tampering.
+
+## Creating an enhanced PrismaClient
+
+Finally, you can call the `enhance` API to create an enhanced PrismaClient with the user information and enjoy automatically enforced access policies.
+
+```ts
+const user = await getCurrentUser(); // your implementation
+const db = enhance(prisma, { user });
+```