Starting out

This video offers a great example of how: “In order to combine multi-tag match, the tags field must be a map” and we must merge results and remove duplicates on client side after running individual queries.

Client Side

But there wasn’t any example for client side code to do so, until I found a plausible suggestion on stackoverflow.

Bind two Observables using the rxjs merge operator

import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/merge';

...

getCombinatedStatus(): Observable<any> {
  return Observable.merge(
    this.db.collection('foo', ref =>
      ref.where('tags.physics','==','true')).valueChanges(),
    this.db.collection('foo', ref =>
      ref.where('tags.chemistry','==','true')).valueChanges());
}

Then subscribe to the new Observable updates using the getCombinatedStatus() method.

getCombinatedStatus.subscribe(results => console.log(results);

Server Side “niche” Workaround

There was another great solution if it fits your dataset. Also a blog discussing the same strategy. And a github issue on the topic.

The only possible values for ours are A,B,C,D (4) so we have to query for things like A||B, A||C, A||B||C etc

So if something only qualified for conditional A of our 4 conditionals (A,B,C,D) then its array would contain the following literal strings:

tags: [
  "A",
  "A||B",
  "A||C",
  "A||D",
  "A||B||C",
  "A||B||D",
  "A||C||D",
  "A||B||C||D"
 ]

Then for any of those OR combinations we can just search array-contains on whatever we may want (e.g. A||C)

I assume some work (worker and client) would go into handling case-sensitivity and alphabetical sorting, both when creating, deleting, updating the tags … and especially when querying/reading them.

Brainstorming

How about assigning each tag a permanent prime number when i its created and using product of two or more such tags as the combination being placed inside tags[] or tags{}?

References

• https://youtu.be/35RlydUf6xo?t=198
• https://stackoverflow.com/questions/46632042/how-to-perform-compound-queries-with-logical-or-in-cloud-firestore#answer-48832819
• https://stackoverflow.com/questions/46632042/how-to-perform-compound-queries-with-logical-or-in-cloud-firestore#answer-55151765
• https://github.com/firebase/firebase-js-sdk/issues/1777
• https://blog.oasisdigital.com/2018/querying-without-or-in-firestore/#post-2022-_adhx5di43dpw
• https://medium.com/google-developer-experts/performing-or-queries-in-firebase-cloud-firestore-for-javascript-with-rxjs-c361671b201e

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