mat-table angular 6 not showing any data despite having the dataSource

Question

I want to show a list through Angular material design's data table mat-table. Here is my component:

import { LeaveapplicationService } from './../../services/leaveapplication.service';
import { leaveapplication } from './../../models/leaveapplication';
import { Component, OnInit, ViewChild, Input } from '@angular/core';
import { MatTableDataSource, MatPaginator, MatSort } from '@angular/material';
// import { DataSource } from '@angular/cdk/table';

@Component({
  selector: 'app-leaveapplication-list',
  templateUrl: './leaveapplication-list.component.html',
  styleUrls: ['./leaveapplication-list.component.scss']
})
export class LeaveapplicationListComponent implements OnInit {
  leaveApplications: leaveapplication[];
  displayedColumns: ['id', 'applicant', 'manager', 'leavetype', 'start', 'end', 'return', 'status'];
  dataSource: MatTableDataSource<leaveapplication>;

  constructor(private leaveApplicationService: LeaveapplicationService) { }

  @ViewChild(MatPaginator) paginator: MatPaginator;
  @ViewChild(MatSort) sort: MatSort;

  ngOnInit() {
    this.leaveApplicationService.getLeaveApplications().subscribe(leaveapplications => {
      this.leaveApplications = leaveapplications;
      this.dataSource = new MatTableDataSource<leaveapplication>(this.leaveApplications);
      this.dataSource.paginator = this.paginator;
      this.dataSource.sort = this.sort;
      console.log(this.dataSource);
    });

  }

  applyFilter(filterValue: string) {
    filterValue = filterValue.trim(); // Remove whitespace
    filterValue = filterValue.toLowerCase(); // MatTableDataSource defaults to lowercase matches
    this.dataSource.filter = filterValue;
  }
}

The leaveapplication interface:

export interface leaveapplication {
  id: number; 
  applicant: string;
  manager: string;
  startdate: Date;
  enddate: Date;
  returndate: Date;
  leavetype: string; 
  status: string;
}

I am getting the dataSource properly in the console:

But the mat-table return empty cells:

Here is my template:

<mat-form-field>
  <input matInput (keyup)="applyFilter($event.target.value)" placeholder="Filter">
</mat-form-field>
<mat-table #table [dataSource]="dataSource" matSort class="mat-elevation-z8">
  <ng-container matColumnDef="id">
    <th mat-header-cell *matHeaderCellDef mat-sort-header> Form ID </th>
    <td mat-cell *matCellDef="let leaveapplication"> {{leaveapplication.id}} </td>
  </ng-container>
  <ng-container matColumnDef="applicant">
    <th mat-header-cell *matHeaderCellDef mat-sort-header> Applicant </th>
    <td mat-cell *matCellDef="let leaveapplication"> {{leaveapplication.applicant}} </td>
  </ng-container>

  <ng-container matColumnDef="manager">
    <th mat-header-cell *matHeaderCellDef mat-sort-header> Manager </th>
    <td mat-cell *matCellDef="let leaveapplication"> {{leaveapplication.manager}} </td>
  </ng-container>

  <ng-container matColumnDef="leavetype">
    <th mat-header-cell *matHeaderCellDef mat-sort-header> Leave Type </th>
    <td mat-cell *matCellDef="let leaveapplication"> {{leaveapplication.leaveType}} </td>
  </ng-container>
  <ng-container matColumnDef="start">
    <th mat-header-cell *matHeaderCellDef mat-sort-header> Start Date </th>
    <td mat-cell *matCellDef="let leaveapplication"> {{leaveapplication.startDate | date:'dd-MM-yyyy'}} </td>
  </ng-container>
  <ng-container matColumnDef="end">
    <th mat-header-cell *matHeaderCellDef mat-sort-header> End Date </th>
    <td mat-cell *matCellDef="let leaveapplication"> {{leaveapplication.endDate | date:'dd-MM-yyyy'}} </td>
  </ng-container>
  <ng-container matColumnDef="return">
    <th mat-header-cell *matHeaderCellDef mat-sort-header> Return Date </th>
    <td mat-cell *matCellDef="let leaveapplication"> {{leaveapplication.returnDate | date:'dd-MM-yyyy'}} </td>
  </ng-container>
  <ng-container matColumnDef="status">
    <th mat-header-cell *matHeaderCellDef mat-sort-header> Status </th>
    <td mat-cell *matCellDef="let leaveapplication"> {{leaveapplication.status}} </td>
  </ng-container>
  <tr mat-header-row *matHeaderRowDef="displayedColumns"></tr>
  <tr mat-row *matRowDef="let row; columns: displayedColumns;"></tr>
</mat-table>

<mat-paginator [pageSizeOptions]="[5, 10, 20]" showFirstLastButtons></mat-paginator>

Why am I not getting the mat-table populated? Is it because of the asynchronous nature of the response?

Answer 1

Hi i have a similar issue, where i can log the data of the DataSource but the Data Table, doesn't display the data.

I created an Array of Mock-Data in my service and passed this instead of the result i get from my database. With the Mock-Array it worked and i got data populated in my mat-table.

After i logged both, the result from the database and the Mock-Data, in the console i noticed that i get an object with an array inside from my database instead of just an array.

Browser console ->

MOCK-DATA: [ {...} ]

DATABASE: { [ {...} ] }

If you look closely you can see in your browser DOM, the data source is set to [Object object] and i think there is the problem, because the Mat-Table can't access the data.

I found a workaround, where i pass the array of the object instead of the object, which is a bit dirty, but does the job until i find the root of the issue.

Here is my service:

rfc.service.ts

@Injectable({
  providedIn: 'root'
})
export class RfcService {
  rfcsChanged = new Subject<Rfc[]>();
  private dbSubs: Subscription[] = [];

  <<MOCK-DATA>>

  constructor(private http: HttpClient) { }

  httpOptions = {
    headers: new HttpHeaders({
      'Content-Type': 'application/json'
    })
  };

  fetchRfcs() {...}

  cancelSubscriptions() {
    this.dbSubs.forEach(sub => sub.unsubscribe());
  }

After i changed my code from:

fetchRfcs() {
  this.dbSubs.push(
    this.http.get<Rfc[]>('http://localhost:3000/rfcs')
     .subscribe((rfcs: Rfc[]) => {
       // console.log(this.MOCK_RFCS);
       // console.log(rfcs);
       this.rfcsChanged.next(rfcs); 
     })
  );
}

to:

fetchRfcs() {
  this.dbSubs.push(
    this.http.get<Rfc[]>('http://localhost:3000/rfcs')
     .subscribe((rfcs: Rfc[]) => {
        // console.log(this.MOCK_RFCS);
        // console.log(rfcs.rfcs);
        this.rfcsChanged.next(rfcs.rfcs); 
     })
    );
}

it worked.

I don't know why my response array is wrapped in an object, but i recommend checking your service and how it receives the data. At the moment i try to find a way to map my response from the backend, so that it isn't wrapped inside an object.

How do you retrieve data in your service?

Answer 2

You have a typo in your component.ts file: You do not assign the array of displayed columns to displayedColumns but instead only define your variable.

So, instead of

 displayedColumns: ['id', 'applicant', 'manager', 'leavetype', 'start', 'end', 'return', 'status'];

it should be

 displayedColumns: string[] = ['id', 'applicant', 'manager', 'leavetype', 'start', 'end', 'return', 'status'];

Answer 3

In the ng-container of the above HTML code change as below.

From matColumnDef="start" to matColumnDef="startDate" From matColumnDef="end" to matColumnDef="endDate " From matColumnDef="return" to matColumnDef="returnDate"

The matColumnDef has to match exactly with the data column, otherwise data wont show up in the mat-table.