What is Firebase Firestore 'Reference' data type good for?

240
votes

I'm just exploring the new Firebase Firestore and it contains a data type called reference. It is not clear to me what this does.

  • Is it like foreign key?
  • Can it be used to point to a collection that is located somewhere else?
  • If reference is an actual reference, can I use it for queries? For example can I have a reference that points directly to the user, instead of storing the userId in a text field? And can I use this user reference for querying?
4
javascriptfirebasegoogle-cloud-firestore
I think this video from firebase team breaks it down for you: youtube.com/watch?v=Elg2zDVIcLo (watch from 4:36) – Adarsh
youtu.be/Elg2zDVIcLo?t=276 – Trevor
I don't like to nest collections in firebase for multiple reasons. If for some reason you had another root level collection that you need to drill all the way down on a sibling root collection; let's say 4 levels to get to a document. This is made a lot easier by using refs and just using db.doc('some_saved_ref') vs matching all the ids out again... from the other root collection. – JustDave

4 Answers

119
votes

References are very much like foreign keys.

The currently released SDKs cannot store references to other projects. Within a project, references can point to any other document in any other collection.

You can use references in queries like any other value: for filtering, ordering, and for paging (startAt/startAfter).

Unlike foreign keys in a SQL database, references are not useful for performing joins in a single query. You can use them for dependent lookups (which seem join like), but be careful because each hop will result in another round trip to the server.

166
votes

Adding below what worked for me using references in Firestore.

As the other answers say, it's like a foreign key. The reference attribute doesn't return the data of the reference doc though. For example, I have a list of products, with a userRef reference as one of the attributes on the product. Getting the list of products, gives me the reference of the user that created that product. But it doesn't give me the details of the user in that reference. I've used other back end as a services with pointers before that have a "populate: true" flag that gives the user details back instead of just the reference id of the user, which would be great to have here (hopefully a future improvement).

Below is some example code that I used to set the reference as well as get the collection of products list then get the user details from the user reference id given.

Set a reference on a collection: 

let data = {
  name: 'productName',
  size: 'medium',
  userRef: db.doc('users/' + firebase.auth().currentUser.uid)
};
db.collection('products').add(data);

Get a collection (products) and all references on each document (user details):

db.collection('products').get()
    .then(res => {
      vm.mainListItems = [];
      res.forEach(doc => {
        let newItem = doc.data();
        newItem.id = doc.id;
        if (newItem.userRef) {
          newItem.userRef.get()
          .then(res => { 
            newItem.userData = res.data() 
            vm.mainListItems.push(newItem);
          })
          .catch(err => console.error(err));
        } else {
          vm.mainListItems.push(newItem);  
        }

      });
    })
    .catch(err => { console.error(err) });

Hope this helps

23
votes

For those looking for a Javascript solution to querying by reference - the concept is that, you need to use a 'document reference' object in the query statement

teamDbRef = db.collection('teams').doc('CnbasS9cZQ2SfvGY2r3b'); /* CnbasS9cZQ2SfvGY2r3b being the collection ID */
//
//
db.collection("squad").where('team', '==', teamDbRef).get().then((querySnapshot) => {
  //
}).catch(function(error) {
  //
});

(Kudos to the answer here: https://stackoverflow.com/a/53141199/1487867)

9
votes

According to the #AskFirebase https://youtu.be/Elg2zDVIcLo?t=276 the primary use-case for now is a link in Firebase console UI