How to perform a query in Firebase's Cloud Firestore

Question

I followed the doc here on how to write a query but I am not getting any data from it. The database is already populated with the example provided by the doc

Below is my code

var db = firebase.firestore();

var citiesRef = db.collection("cities");
var query = citiesRef.where("state", "==", "CA");

query.get().then(function(doc) {
if (doc.exists) {
    console.log("Document data:", doc.data());
} else {
    console.log("No such document!");
}
}).catch(function(error) {
    console.log("Error getting document:", error);
});

It works fine if I didn't put any queries on it. For example (also from the doc):

var docRef = db.collection("cities").doc("SF");

docRef.get().then(function(doc) {
    if (doc.exists) {
        console.log("Document data:", doc.data());
    } else {
        console.log("No such document!");
    }
}).catch(function(error) {
    console.log("Error getting document:", error);
});

Answer 1

The difference in your two requests is that in the second case you are retrieving one document which gives you a DocumentSnapshot which has the exists property and the data() method.

In your not working example you do a query, which gives you a QuerySnapshot that has to be handled differently from a DocumentSnapshot. Instead of a single document you get a list/collection of documents. You can check if data has been retrieved using the empty or size properties, and then go through the results using the forEach method or going through the docs array:

var db = firebase.firestore();

var citiesRef = db.collection("cities");
var query = citiesRef.where("state", "==", "CA");

query.get().then(function(results) {
  if(results.empty) {
    console.log("No documents found!");   
  } else {
    // go through all results
    results.forEach(function (doc) {
      console.log("Document data:", doc.data());
    });

    // or if you only want the first result you can also do something like this:
    console.log("Document data:", results.docs[0].data());
  }
}).catch(function(error) {
    console.log("Error getting documents:", error);
});