How to use md-table with services in Angular 4

I am quite new in the angular world and i'm trying to use the new md-table component in Angular Material 2 with Angular 4.

I've made a service from an API which retrieves simple arrays of content. Now I'm trying to use this service as a data source for the md-table but I can't find a way to get the data from the service (it always return me an empty array).

Please note that before using md-table, I was using already using the service and it worked normally.

Here is the code for the component :

import { Component, OnInit, ViewChild } from '@angular/core';
import {DataSource} from '@angular/cdk';
import {MdPaginator} from '@angular/material';
import {BehaviorSubject} from 'rxjs/BehaviorSubject';
import {Observable} from 'rxjs/Observable';
import 'rxjs/add/operator/startWith';
import 'rxjs/add/observable/merge';
import 'rxjs/add/operator/map';

import { GroupService } from '../shared/group.service';
import { Group } from '../shared/group';

@Component({
  selector: 'app-group-list',
  templateUrl: './group-list.component.html',
  styleUrls: ['./group-list.component.css'],
  providers: [GroupService]
})
export class GroupListComponent implements OnInit{

  public DisplayedColumns = ['name', 'email', 'directMembersCount'];
  public groupDatabase = new GroupDatabase();
  public dataSource : CustomDataSource | any;

  @ViewChild(MdPaginator) paginator : MdPaginator;

  constructor() {}

  ngOnInit() {
    this.dataSource = new CustomDataSource(this.groupDatabase, this.paginator);
    console.log(this.dataSource);
  }

}

export class GroupDatabase implements OnInit {

  public dataChange: BehaviorSubject<Group[]> = new BehaviorSubject<Group[]>([]);
  get data(): Group[] { return this.dataChange.value }

  private _groupService : GroupService

  private getAllGroups(){
    return this._groupService
      .getAllGroups();
  }

  constructor (){}

  ngOnInit() {
      this.getAllGroups();
      console.log(this.getAllGroups());
  }
}

export class CustomDataSource extends DataSource<any> {

  constructor(private _groupDatabase = new GroupDatabase(), private _paginator: MdPaginator){
    super();
  }

  connect(): Observable<Group[]> {
    const displayDataChanges = [
      this._groupDatabase.dataChange,
      this._paginator.page
    ];
    return Observable.merge(...displayDataChanges).map(() => {
      const data = this._groupDatabase.data.slice();
      console.log(data);

      const startIndex = this._paginator.pageIndex * this._paginator.pageSize;
      return data.splice(startIndex, this._paginator.pageSize);
    })
  }

  disconnect() {}
}

Here is the code for the HTML :

<md-table #table [dataSource]="dataSource">

  <ng-container *cdkColumnDef="name">
    <md-header-cell *cdkCellDef>Nom</md-header-cell>
    <md-cell *cdkCellDef="let row"> {{row.name}} </md-cell>
  </ng-container>

  <ng-container *cdkColumnDef="email">
    <md-header-cell *cdkCellDef>Email</md-header-cell>
    <md-cell *cdkCellDef="let row"> {{row.email}} </md-cell>
  </ng-container>

  <ng-container *cdkColumnDef="directMembersCount">
    <md-header-cell *cdkCellDef>Nombre de membres</md-header-cell>
    <md-cell *cdkCellDef="let row"> {{row.directMembersCount}} </md-cell>
  </ng-container>

  <md-header-row *cdkHeaderRowDef="displayedColumns"></md-header-row>
  <md-row *cdkRowDef="let row; columns: DisplayedColumns;"></md-row>

</md-table>

<md-paginator #paginator
              [length]="groupDatabase.data.length"
              [pageIndex]="0"
              [pageSize]="25"
              [pageSizeOptions]="[5, 10, 25, 100]">
</md-paginator>

And the concerned service :

private groupApiUrl: string;
  private groupsApiUrl: string;
  private headers: Headers;
  constructor(public http:Http, private config: Config) {
    this.groupApiUrl = config.serverWithApi + "group";
    this.groupsApiUrl = config.serverWithApi + "groups";

    this.headers = new Headers();
    this.headers.append('Content-Type', 'application/json');
    this.headers.append('Accept', 'application/json');
  }

  public getAllGroups = (): Observable<Group[]> => {
    return this.http.get(this.groupsApiUrl)
      .map((response: Response) => <Group[]>response.json())
      .catch(this.handleError)
  }

I'm not sure how I should call the service using the datasource, that's why I did it as I was doing before; using the ngOnInit method.

Thanks for you help.