Firestore: How to get random documents in a collection

Question

It is crucial for my application to be able to select multiple documents at random from a collection in firebase.

Since there is no native function built in to Fireb

Answer 1

For those using Angular + Firestore, building on @Dan McGrath techniques, here is the code snippet.

Below code snippet returns 1 document.

  getDocumentRandomlyParent(): Observable {
    return this.getDocumentRandomlyChild()
      .pipe(
        expand((document: any) => document === null ? this.getDocumentRandomlyChild() : EMPTY),
      );
  }

  getDocumentRandomlyChild(): Observable {
      const random = this.afs.createId();
      return this.afs
        .collection('my_collection', ref =>
          ref
            .where('random_identifier', '>', random)
            .limit(1))
        .valueChanges()
        .pipe(
          map((documentArray: any[]) => {
            if (documentArray && documentArray.length) {
              return documentArray[0];
            } else {
              return null;
            }
          }),
        );
  }

1) .expand() is a rxjs operation for recursion to ensure we definitely get a document from the random selection.

2) For recursion to work as expected we need to have 2 separate functions.

3) We use EMPTY to terminate .expand() operator.

import { Observable, EMPTY } from 'rxjs';