Listing real time firebase database content

Hello guys,
Currently i have the following structure:

I am trying to list the categories then all the items in those categories, the list has to be grouped by the categories.


> this.firebasedb.list("/tavak/").subscribe(_data => {
>       this.toadatok = _data.filter(item => item.approved == "1");
>     })
>   }

        <ion-item-group *ngFor="let item of toadatok; let i = index">
          <ion-item-divider color="secondary">{{item.$key}}</ion-item-divider>
          <button *ngFor="let c of toadatok ion-item>

My main problem i can list the category name but cant go though the object in the categories.See the console log below

You can try this.firebasedb.list("/tavak/"+"category_name") and will return the items inside of that category.

yes but i need to list all the categories in my page not just the content of one category.
Console log:

Added new info,i list all the objects (categories) in my db, but now i have to list the objects in those categories in a grouped list