Add guide for validating nested form groups in Angular

docs/en/framework/ui/angular/form-validation.md

@@ -178,3 +178,211 @@ export class AppModule {}
 The error message will be bold and italic now:
 
 <img alt="A required field is cleared and a bold and italic error message appears." src="./images/form-validation---custom-error-template.gif" width="990px" style="max-width:100%">
+
+## How to Validate Nested Form Groups
+
+There are multiple ways to validate nested form groups in ABP Angular UI. Below is the first and most common approach, using automatic validation and error messages with nested reactive forms. (A second method will be described in the next section.)
+
+### 1st Way: Automatic Validation and Error Message Using Nested Reactive Forms
+
+ABP Angular UI leverages Angular's reactive forms and the [ngx-validate](https://www.npmjs.com/package/@ngx-validate/core) library to provide a robust, flexible, and user-friendly form validation experience. Whether you build your forms manually or use ABP’s dynamic form generation features, validation and error messages are handled automatically.
+
+#### Key Features
+
+- **Automatic Validation:**  
+  All validation rules defined in your DTOs (such as `[Required]`, `[StringLength]`, `[EmailAddress]`, etc.) are automatically reflected in the Angular form. Error messages are shown under each field without any extra markup.
+
+- **Nested Form Groups and Dynamic Fields:**  
+  For complex data structures, you can group fields or manage dynamic lists using nested `FormGroup` and `FormArray` structures. Validation and error display work seamlessly for both parent and child controls.
+
+- **Dynamic and Extensible Forms:**  
+  With ABP’s extensibility system, you can generate forms dynamically using helpers like `generateFormFromProps` and display them with the `abp-extensible-form` component. This ensures all entity properties (including extension properties) are included in the form and their validation rules are applied.
+
+- **No Extra Boilerplate:**  
+  You do not need to add custom error components or directives for validation. The system works out of the box, including for nested and dynamically generated controls.
+
+#### Real-World Example: Nested Form Groups in the Users Form
+
+Below is a real example from the Users management form in ABP Angular UI, showing how nested form structures and validation are implemented. This example includes both dynamically generated fields (with `abp-extensible-form`) and a dynamic list of roles using `FormArray` and `FormGroup`.
+
+**TypeScript: Building the Form**
+
+```ts
+buildForm() {
+  const data = new FormPropData(this.injector, this.selected);
+  this.form = generateFormFromProps(data); // Automatically creates form controls from entity and extension properties
+
+  this.service.getAssignableRoles().subscribe(({ items }) => {
+    this.roles = items;
+    if (this.roles) {
+      // Dynamic roles list: nested FormArray and FormGroup
+      this.form.addControl(
+        'roleNames',
+        this.fb.array(
+          this.roles.map(role =>
+            this.fb.group({
+              [role.name as string]: [
+                this.selected?.id
+                  ? !!this.selectedUserRoles?.find(userRole => userRole.id === role.id)
+                  : role.isDefault,
+              ],
+            }),
+          ),
+        ),
+      );
+    }
+  });
+}
+```
+
+**HTML: Displaying the Form**
+
+```html
+<abp-modal [(visible)]="isModalVisible" [busy]="modalBusy">
+  <ng-template #abpHeader>
+    <h3>{{ (selected?.id ? 'AbpIdentity::Edit' : 'AbpIdentity::NewUser') | abpLocalization }}</h3>
+  </ng-template>
+
+  <ng-template #abpBody>
+    @if (form) {
+      <form [formGroup]="form" (ngSubmit)="save()">
+        <ul ngbNav #nav="ngbNav" class="nav-tabs">
+          <li ngbNavItem>
+            <a ngbNavLink>{{ 'AbpIdentity::UserInformations' | abpLocalization }}</a>
+            <ng-template ngbNavContent>
+              <!-- Automatically displays all entity fields and their validation -->
+              <abp-extensible-form [selectedRecord]="selected"></abp-extensible-form>
+            </ng-template>
+          </li>
+
+          <li ngbNavItem>
+            <a ngbNavLink>{{ 'AbpIdentity::Roles' | abpLocalization }}</a>
+            <ng-template ngbNavContent>
+              <!-- Dynamic roles list: nested FormArray and FormGroup -->
+              @for (roleGroup of roleGroups; track $index; let i = $index) {
+                <div class="form-check mb-2">
+                  <abp-checkbox
+                    [formControl]="roleGroup.controls[roles[i].name]"
+                    [label]="roles[i].name"
+                  ></abp-checkbox>
+                </div>
+              }
+            </ng-template>
+          </li>
+        </ul>
+        <div class="mt-2 fade-in-top" [ngbNavOutlet]="nav"></div>
+      </form>
+    } @else {
+      <div class="text-center"><i class="fa fa-pulse fa-spinner" aria-hidden="true"></i></div>
+    }
+  </ng-template>
+</abp-modal>
+```
+
+**Explanation:**
+- `abp-extensible-form` automatically generates and displays all entity fields and their validation.
+- In the Roles tab, each role is represented by a checkbox, and these checkboxes are managed in a `FormArray`, with each as a `FormGroup`. This is a real-world example of a nested form structure.
+- All validation and error messages are shown automatically for both the main form and nested groups.
+
+
+### 2nd Way: Manual Nested Reactive Forms Without abp-extensible-form
+
+You can also build and validate nested form groups manually, without using `abp-extensible-form` or dynamic helpers. This approach gives you full control over the form structure and is useful for custom or non-entity-based forms.
+
+#### Example: Simple Manual Nested FormGroup
+
+Below is a simple, generic example of a nested reactive form. This form includes a nested `FormGroup` for profile information and demonstrates how to apply validation rules.
+
+**TypeScript: Building the Form**
+
+```ts
+import { Component, OnInit, inject } from '@angular/core';
+import { FormBuilder, FormGroup, Validators } from '@angular/forms';
+import { NgxValidateCoreModule } from '@ngx-validate/core';
+
+@Component({
+  selector: 'app-nested-form',
+  templateUrl: './nested-form.component.html',
+  standalone: true,
+  imports: [NgxValidateCoreModule],
+})
+export class NestedFormComponent implements OnInit {
+  form: FormGroup;
+
+  private fb = inject(FormBuilder);
+
+  ngOnInit() {
+    this.buildForm();
+  }
+
+  buildForm() {
+    this.form = this.fb.group({
+      userName: ['', Validators.required],
+      email: ['', [Validators.required, Validators.email]],
+      profile: this.fb.group({
+        firstName: ['', Validators.required],
+        lastName: ['', Validators.required],
+      }),
+    });
+  }
+
+  submit() {
+    if (this.form.invalid) {
+      return;
+    }
+    // handle submit
+  }
+}
+```
+
+**HTML: Displaying the Form**
+
+```html
+<form [formGroup]="form" (ngSubmit)="submit()">
+  <div class="mb-3">
+    <label class="form-label">User Name</label>
+    <input type="text" class="form-control" formControlName="userName" />
+  </div>
+
+  <div class="mb-3">
+    <label class="form-label">Email</label>
+    <input type="email" class="form-control" formControlName="email" />
+  </div>
+
+  <div formGroupName="profile" class="card mt-3">
+    <div class="card-header">
+      <strong>Profile Details</strong>
+    </div>
+    <div class="card-body">
+      <div class="mb-3">
+        <label class="form-label">First Name</label>
+        <input type="text" class="form-control" formControlName="firstName" />
+      </div>
+
+      <div class="mb-3">
+        <label class="form-label">Last Name</label>
+        <input type="text" class="form-control" formControlName="lastName" />
+      </div>
+    </div>
+  </div>
+
+  <hr class="my-3" />
+
+  <div>
+    <abp-button buttonType="submit" iconClass="fa fa-save" [disabled]="form.invalid">
+      Save
+    </abp-button>
+  </div>
+</form>
+```
+
+**How it works:**
+- The form contains main fields (`userName`, `email`) and a nested `FormGroup` (`profile`).
+- The `profile` group includes `firstName` and `lastName` fields, each with their own validation rules.
+- Validation rules are defined directly in the form builder.
+- Error messages and validation feedback are handled automatically by ngx-validate and ABP Angular UI, just like with dynamic forms.
+- This structure ensures that validation works automatically for both the main form and nested groups.
+
+> **Note:** This approach is ideal for custom forms or when you want full control over the form structure. It provides a user experience and validation behavior similar to ABP's dynamic forms, but with manual control over the form layout and logic.
+
+---