angular 5 установить значение для вложенного массива формы группы форм

я открываю stackblitz, вы можете попробовать что-то вроде этого:

product.component.ts:

import { Component, Input} from '@angular/core';
import {FormGroup} from "@angular/forms";


@Component({
  selector: 'product',
  templateUrl: './product.component.html'
})
export class ProductComponent {
  @Input('group')
  public productForm: FormGroup;
}

product.component.html:

<div [formGroup]="productForm">
  <div class="form-group col-xs-6">
    <label>id</label>
    <input type="text" class="form-control" formControlName="id">
    <small [hidden]="productForm.controls.id.valid" class="text-danger">
      id is required
    </small>
  </div>
  <div class="form-group col-xs-6">
    <label>productId</label>
    <input type="text" class="form-control" formControlName="productId">
  </div>
  <div class="form-group col-xs-6">
    <label>item</label>
    <input type="text" class="form-control" formControlName="item">
  </div>
</div>

app.component.ts:

import { Component} from '@angular/core';
import {FormArray, FormBuilder, FormGroup, Validators} from "@angular/forms";


@Component({
  selector: 'app-root',
  templateUrl: './app.component.html'
})
export class AppComponent {

  public myForm: FormGroup;

  constructor(private _fb: FormBuilder) { }

  ngOnInit() {
    this.myForm = this._fb.group({
      name: ['', [Validators.required, Validators.minLength(5)]],
      products: this._fb.array([])
    });

    this.addProduct();
  }

  initProduct() {
    return this._fb.group({
      id: ['', Validators.required],
      productId: [''],
      item: ['']
    });
  }

  addProduct() {
    const control = <FormArray>this.myForm.controls['products'];
    const productCtrl = this.initProduct();
    control.push(productCtrl);
  }

  removeProduct(i: number) {
    const control = <FormArray>this.myForm.controls['products'];
    control.removeAt(i);
  }

  save(model: Product) {
    console.log(model);
  }
}

export interface Products {
  name: string;
  products: Product[];
}

export interface Product {
  id: string;
  productIid: string;
  item:string;
}

app.component.html:

<div class="container">
  <div class="row">
    <div class="col-md-12">
      <div class="margin-20">
        <h4>Add Product</h4>
      </div>
      <form [formGroup]="myForm" novalidate (ngSubmit)="save(myForm)">
        <div class="form-group">
          <label>Name</label>
          <input type="text" class="form-control" formControlName="name">
          <small *ngIf="!myForm.controls.name.valid" class="text-danger">
            Name is required (minimum 5 characters).
          </small>
        </div>

        <div formArrayName="products">
          <div *ngFor="let product of myForm.controls.products.controls; let i=index" class="panel panel-default">
            <div class="panel-heading">
              <span>Product {{i + 1}}</span>
              <span class="glyphicon glyphicon-remove pull-right" *ngIf="myForm.controls.products.controls.length > 1" (click)="removeProduct(i)">remove</span>
            </div>
            <div class="panel-body" [formGroupName]="i">
              <product [group]="myForm.controls.products.controls[i]"></product>
            </div>
          </div>
        </div>

        <div class="margin-20">
          <a (click)="addProduct()" style="cursor: default">
            Add another product +
          </a>
        </div>

        <div class="margin-20">
          <button type="submit" class="btn btn-primary pull-right" [disabled]="!myForm.valid">Submit</button>
        </div>
        <div class="clearfix"></div>

        <div class="margin-20">
          <div>myForm details:-</div>
          <pre>Is myForm valid?: <br>{{myForm.valid | json}}</pre>
          <pre>form value: <br>{{myForm.value | json}}</pre>
        </div>
      </form>
    </div>
  </div>
</div>

и, наконец, app.module.ts:

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { ProductComponent} from "./product.component";
import { ReactiveFormsModule , FormsModule } from "@angular/forms";


@NgModule({
  declarations: [
    AppComponent,
    ProductComponent,

  ],
  imports: [
    BrowserModule,
    ReactiveFormsModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

надеюсь это поможет.