collectionGroup(collectionId, queryGroupFn) {
        const queryFn = queryGroupFn || (ref => ref);
        const collectionGroup = this.firestore.collectionGroup(collectionId);
        return new AngularFirestoreCollectionGroup(queryFn(collectionGroup), this);
    }
/**
     * Create a reference to a Firestore Collection Group based on a collectionId
     * and an optional query function to narrow the result
     * set.
     */
Create a reference to a Firestore Collection Group based on a collectionId
and an optional query function to narrow the result
set.
Create a reference to a Firestore Collection Group based on a collectionId and an optional query function to narrow the result set .
collectionGroup(collectionId, queryGroupFn) {
        const queryFn = queryGroupFn || (ref => ref);
        const collectionGroup = this.firestore.collectionGroup(collectionId);
        return new AngularFirestoreCollectionGroup(queryFn(collectionGroup), this);
    }
collectionGroup ( collectionId , queryGroupFn ) { const queryFn = queryGroupFn || ( ref => ref ) ; const collectionGroup = this . firestore . collectionGroup ( collectionId ) ; return new AngularFirestoreCollectionGroup ( queryFn ( collectionGroup ) , this ) ; }
Create a reference to a Firestore Collection Group based on a collectionId
 and an optional query function to narrow the result
 set.
Create a reference to a Firestore Collection Group based on a collectionId and an optional query function to narrow the result set .