docs: clarify generateId behavior and mixed ID types (#5173)

Co-authored-by: Maxwell <145994855+ping-maxwell@users.noreply.github.com>

docs/content/docs/concepts/database.mdx

@@ -471,11 +471,11 @@ export const auth = betterAuth({
 
 ### ID Generation
 
-Better Auth by default will generate unique IDs for users, sessions, and other entities. If you want to customize how IDs are generated, you can configure this in the `advanced.database.generateId` option in your auth config.
+Better Auth by default will generate unique IDs for users, sessions, and other entities. You can customize ID generation behavior using the `advanced.database.generateId` option or `advanced.database.useNumberId`.
 
-You can also disable ID generation by setting the `advanced.database.generateId` option to `false`. This will assume your database will generate the ID automatically.
+#### Option 1: Let Database Generate IDs
 
-**Example: Automatic Database IDs**
+Setting `generateId` to `false` disables ID generation globally, letting your database handle all ID generation:
 
 ```ts title="auth.ts"
 import { betterAuth } from "better-auth";
@@ -485,13 +485,48 @@ export const auth = betterAuth({
   database: db,
   advanced: {
     database: {
-      generateId: false,
+      generateId: false, // Database generates all IDs
     },
   },
 });
 ```
 
-**Example: Using a Custom ID Generator**
+<Callout type="info">
+  **Note**: For auto-incrementing numeric IDs (serial, auto_increment), use `useNumberId: true` instead (see the Numeric IDs section below).
+</Callout>
+
+#### Option 2: Custom ID Generation Function
+
+Use a function to generate IDs. You can return `false` or `undefined` from the function to let the database generate the ID for specific models:
+
+```ts title="auth.ts"
+import { betterAuth } from "better-auth";
+import { db } from "./db";
+
+export const auth = betterAuth({
+  database: db,
+  advanced: {
+    database: {
+      generateId: (options) => {
+        // Let database auto-generate for specific models
+        if (options.model === "user" || options.model === "users") {
+          return false; // Let database generate ID
+        }
+        // Generate UUIDs for other tables
+        return crypto.randomUUID();
+      },
+    },
+  },
+});
+```
+
+<Callout type="info">
+  **Important**: Returning `false` or `undefined` from the `generateId` function lets the database handle ID generation for that specific model. Setting `generateId: false` (without a function) disables ID generation for **all** tables.
+</Callout>
+
+#### Option 3: Consistent Custom ID Generator
+
+Generate the same type of ID for all tables:
 
 ```ts title="auth.ts"
 import { betterAuth } from "better-auth";
@@ -538,6 +573,50 @@ export const auth = betterAuth({
   this is normal. It's also expected that all id values passed to Better-Auth (eg via an endpoint body) is expected to be a string.
 </Callout>
 
+### Mixed ID Types
+
+If you need different ID types across tables (e.g., integer IDs for users, UUID strings for sessions/accounts/verification), use a `generateId` function instead of `useNumberId`.
+
+<Callout type="warn">
+  **Do NOT use `useNumberId` when mixing ID types**. The `useNumberId` option is global and affects all tables. It will disable internal ID generation and expect all IDs to be numbers.
+</Callout>
+
+**Example: Integer IDs for Users, UUIDs for Other Tables**
+
+```ts title="auth.ts"
+import { betterAuth } from "better-auth";
+import { db } from "./db";
+
+export const auth = betterAuth({
+  database: db,
+  user: {
+    modelName: "users", // PostgreSQL: id serial primary key
+  },
+  session: {
+    modelName: "session", // PostgreSQL: id text primary key
+  },
+  advanced: {
+    database: {
+      // Do NOT set useNumberId - it's global and affects all tables
+      generateId: (options) => {
+        if (options.model === "user" || options.model === "users") {
+          return false; // Let PostgreSQL serial generate it
+        }
+        return crypto.randomUUID(); // UUIDs for session, account, verification
+      },
+    },
+  },
+});
+```
+
+This configuration allows you to:
+- Use database auto-increment (serial, auto_increment, etc.) for the users table
+- Generate UUIDs for all other tables (session, account, verification)
+- Maintain compatibility with existing schemas that use different ID types
+
+<Callout type="info">
+  **Use Case**: This is particularly useful when migrating from other authentication providers (like Clerk) where you have existing users with integer IDs but want UUID strings for new tables.
+</Callout>
 
 ### Database Hooks
 
@@ -616,6 +695,7 @@ export const auth = betterAuth({
 });
 ```
 
+
 #### Throwing Errors
 
 If you want to stop the database hook from proceeding, you can throw errors using the `APIError` class imported from `better-auth/api`.