如何删除angularfire2中的链接引用？

Question

I have three models City (have multiple Block s), Block (have multiple Area s) and Area (the one to many relation continues to other models, not showing here).

From the picture you can see I have flatten the data and using $key references.

My question is what is the best way to store this data and how to delete all linked references? Say when one City is deleted all the references of Block and all the Area linked with Block should be deleted.

Below is the data structure

The code I am right now using to delete is, not upto Area deletion:

    deleteCity(city: City): void {
      this.af.database.list(`/cities/${city.$key}/blocks`, { preserveSnapshot: true })
                .subscribe(snapshots => {
                    snapshots.forEach(snapshot => {
                          this.af.database.object(`/blocks/${snapshot.key}`).remove();
                        });
              });
       this.af.database.list('/cities').remove(city.$key).then(
            () => this.appstore.dispatch({ type: DELETE_CITY, payload: city})
    );
  }

Please suggest about the data design and also deletion process.

Answer 1

The first thing you should probably do is wait until the blocks have been removed before removing the city:

import 'rxjs/add/operator/first';
import 'rxjs/add/operator/toPromise';

// Use the first operator to take the first emitted list and then
// complete the observable and use the toPromise operator to
// convert it into a promise for simpler chaining.

deleteCity(city: City): void {
  this.af.database
    .list(`/cities/${city.$key}/blocks`, { preserveSnapshot: true })
    .first()
    .toPromise()
    .then(snapshots => {
      snapshots.forEach(snapshot => {
        this.af.database.object(`/blocks/${snapshot.key}`).remove();
      });
    })
    .then(() => this.af.database.list('/cities').remove(city.$key))
    .then(() => this.appstore.dispatch({ type: DELETE_CITY, payload: city});
);

You should do something similar for areas, too.

Beyond that, any changes to the database structure the data depend upon how you want to access it.

If you only need to access blocks in the context of a city and areas in the context of a block and never need to access arbitrary blocks or areas, you could include the city key in their paths:

/areas/$cityKey/$areaKey
/blocks/$cityKey/$blockKey

Removing all of the blocks and areas for a city, etc. is then trivial.

Another option would be to use the cityKey child property that you have in each block. You can grab those with a query like this:

this.af.database
  .list(`/blocks`, {
    preserveSnapshot: true,
    query: {
      orderByChild: 'cityKey',
      equalTo: '-KaXa...'
    }
  })

Similarly, if you add a cityId child to areas, you could query those in a similar manner. That would at least avoid having to get all of the blocks for a city and all of the areas for each block; you'd just get all of the blocks and areas using the city key.