I think the this would be a good solution if your intention to know the record's id when querying the collection.

    firebase.firestore().collection('sets').get()
        .then(querySnapshot => {
          querySnapshot.forEach(doc => {
            this.sets.push({ ...doc.data(), id: doc.id })
          })
        })
        .catch(error => {
          console.log(error)
        })