Understanding how queries work on the Firestore Database is one of the valuable skills to have as a Firebase Developer such as the Cloud Firestore is getting more popular than the real-time database.

In this Firestore tutorial, I will be covering how to do a simple CRUD (Create, Read, Update and Delete) operations with Firestore Database.

After that, you are going to learn how to make queries using WHERE (single/multiple), ORDERBY and LIMIT filters.

Then, I will guide you through how to get sub-collections data Collection Group queries which is one of the new features at the time of this post.

Finally, I am going to be teaching you how to split queries when you have a large collection of documents using pagination with Query Cursor function for a better experience and to save money.

Sounds interesting! 😯… A lot to cover!

Let’s get started. 🚀

Feel free to jump into any of the sections below.

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Add/Create A Document To Cloud Firestore

There are two ways to create/add a new document to the Cloud Firestore, which are:

add()

set()

Let’s take a look at add() method sample code.

const db = firebase.firestore() db.collection("users").add({ name: "Anbu Selvan", email: "anbu.selvan@email.com", age: 25 })

To run the sample code in your project, you will need to add Firebase to your project.

First, get a reference to the Firestore database using firestore() method and store it in db .

Then, obtain a reference to the users collection by invoking collection() method on the db object.

Finally, run add() method by passing new data as a JavaScript object.

That’s it.

Let’s take a look at set() method.

As you can see below, the set() method is very similar to add() .

db.collection("users") .doc() .set({ name: "Anbu Selvan", email: "anbu.selvan@email.com", age: 25 }) ﻿

The only difference is that you can add a document to a collection directly using add() method, but for the set() method you need to explicitly specify the document identifier by invoking doc().

If you do not specify a document identifier, it will be created.

Under the hood, both set() and add() are working very similar according to the Firestore Documentation.

I prefer to use set() over add() because I can use a single method to add or update data. 🙂

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There are two options to update existing data.

set()

update()

Overwriting A Document using set()

When you want to overwrite or completely replace an existing document, you can easily do that by using the set() method by passing an existing auto-generated document identifier as an argument to the doc() method.

db.collection("users") .doc("3P86VJxcpBK0D0lsAyYx") .set({ name: "Lee Kuan", });

Overriding A Document using set()

There are some cases where you will need to just update (override) one or more fields rather than replacing the whole document.

This can also be done by set() method as well.

db.collection("users") .doc("3P86VJxcpBK0D0lsAyYx") .set( { name: "Anbu Selvan", age: 25 }, { merge: true } );

The above code is very similar to the previous one, with the only difference being it has a JavaScript object {merge: true} as a second argument to the set() method which will prevent overwriting an entire document.

At this stage, the set() method will only update with new values to the targetted document if the name and age fields exist.

Otherwise, the missing fields will be created.

In this case, the value of name will be replaced from Lee Kuan to Anbu Selvan and the age field will be added to the document as it did not exist before.

Overriding A Document using update()

The update() method is very similar to set() without the second argument and is pretty straight forward.

db.collection("users") .doc("3P86VJxcpBK0D0lsAyYx") .update( { name: "Anbu Selvan", email: "anbu.selvan@email.com", }, );

You must provide an auto-generated ID as an argument to the doc() when using update() method to have it work.

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Delete Document from Cloud Firestore

Delete A Field From A Document on the Firestore Database

You can delete an entire document from Cloud Firestore using delete() method by passing its auto-generated ID as an argument to the doc() method.

db.collection("users") .doc("3P86VJxcpBK0D0lsAyYx") .delete() .then(function () { console.log("Document successfully deleted!"); }).catch( function(error) { console.error("Error removing document: ", error); }); ﻿

One more thing I want to point out here is that sub-collections won’t be deleted when you delete the parent document.

Delete A Field From A Document on the Firestore Database

To delete a specific field from a document, use update() method and inside it add the field that you want to delete as a javascript object and set firebase.firestore.FieldValue.delete() as a value of it.

db.collection("users") .doc("3P86VJxcpBK0D0lsAyYx") .update({ email.firestore.FieldValue.delete() }) ﻿

Pretty straight forward.

Before getting into retrieving data from the Firestore Database, let’s add some data to the Firestore Database calling the following function once.

addUsersToFirestore() { var users = [{ name: "Raja", email: "raja.tamil@email.com", createdAt: new Date("2019-01-01 12:08:00") }, { name: "Arivu", email: "arivu.selvan@email.com", createdAt: new Date("2018-01-23 09:13:00") }, { name: "Mike", email: "mike.author@email.com", createdAt: new Date("2018-08-08 06:37:00") }, { name: "Praba", email: "praba.karan@email.com", createdAt: new Date("2018-10-09 18:26:00") }, { name: "Muhammad", email: "muhammad.ali@email.com", createdAt: new Date("2018-03-13 12:13:00") } ]; const db = firebase.firestore(); users.forEach(user => { db.collection("users").doc().set(user); }); }

If everything goes well, you should have a collection called users with five documents.

Add Users Collection to Cloud Firestore Database

Get Documents Data from Firestore Database

There are a couple of options for how you can get or retrieve data from the Cloud Firestore Database.

get(): This method will be invoked and get data only once until it’s called again.

This method will be invoked and get data only once until it’s called again. onSnapShot(): Unlike get() method, this method will be triggered every time data changes in a location that it’s listening for.

In the code below, I use the get() method on the users collection reference to get all the documents data from it.

It will return a promise, and if the promise is full-filled the then() function will be called and the callback function will have a snapshot object snap .

db.collection("users") .get() .then(snap => { snap.forEach(doc => { console.log(doc.data()); console.log(doc.id); }); });

The snap object contains all the user documents from the users collection with some other meta information.

You can get an individual document using forEach() by passing a callback function on each iteration.

On each iteration, you will get value named doc , which is also a firebase query snapshot, that contains each user document with some additional metadata.

Inside that loop, you can invoke data() method on the doc object to get an actual user document.

Also, you can get the auto-id of a user using id property on the doc object.

At this stage, if you make any changes on the users collection, you will have to refresh the page in order to see the change as the get() method will be called once.

To see a real-time change on the view, you will need to use onSnapShot() listener.

db.collection("users") .onSnapshot() .then(snap => { snap.forEach(doc => { console.log(doc.data()); }); });

As you can see, the above code is almost identical to the previous one. The only change is replacing the get() to onSnapShot() method.

Using the onSnapShot() listener method, it will update the view automatically when data changes on the Firestore Database which is cool! 😎

Get A Single Document Data

To get a specific document, call doc() method by passing the auto-generated document ID to it.

db.collection("users") .doc("cAwTiq7IYKAbFGnhgKT3") .get() .then(doc => { console.log(doc.data()) })

firestore-get-a-single-document

Make sure that the get() method always gets run last.

Get Data from Sub-collection in Firestore

Sub-collections are a great way to structure your data.

As you can see, I have a users collection that has five documents in it.

Each user document has a sub-collection called posts which has three fields: author, title, and publishedAt.

Firestore queries are shallow.

So, when you query to get all the user documents form users collection, you won’t get any sub-collections underneath them.

To get all documents from a sub-collection of a specific user, you can query like this:

db.collection("users") .doc("cAwTiq7IYKAbFGnhgKT3") .collection("posts") .get() .then(snap => { snap.forEach(doc => { console.log(doc.data()); }); });

If you want to get a specific document from a sub-collection of a specific user, the query should be like this:

db.collection("users") .doc("cAwTiq7IYKAbFGnhgKT3") .collection("posts") .doc("BjLZHiuQfVQVOu9nEG7k") .get() .then(snap => { console.log(snap.data()); });

The one restriction here is that there is no way to retrieve all documents from the users collection as well as orders collection all at once. If you’re trying to make a query like that, you might want to think about changing your data structure.

Firestore Single/Multiple Where Query Filter

Querying Firestore with the where clause is one of the most common ways to filter documents using query operators == <, , <= , > , >= , etc.

Let’s see how to make a query with a single where clause.

For example, if I want to get users where their age is equal to or less than 30, my query will look like this:

db.collection("users") .where("age", "<=", 30) .get() .then(snap => { snap.forEach(doc => { console.log(doc.data()); }); });

As you can see, the where() method takes three arguments which are a field name, relational operator and the value that you want to compare against to the field name. These arguments look very similar to an if condition block.

Voila!

Now, let’s make a query with multiple where clauses.

For that, I want documents where the age is less than or equal to 30 but greater than or equal to 20.

db.collection("users") .where("age", "<=", 30) .where("age", ">=", 20) .get() .then(snap => { snap.forEach(doc => { console.log(doc.data()); }); });

That was easy!

Sometimes, you will need to create a composite index to get the query working.

Let’s say you want to get the documents from users collection where each user age is greater than or equal to 20 and gender is equal to female.

db.collection("users") .where("age", ">=", 20) .where("gender", "==", "fmale") .get() .then(snap => { snap.forEach(doc => { console.log(doc.data()); }); });

When you use equal operator and range operator in a single query, you will need to create a composite index.

The quickest way to create an index for this query is to go to the debug area on the browser and click the link that is provided by Firebase.

It will take you to your Firebase Console Dashboard -> Database -> Create A Composite Index.

And click Create Index.

This will take several minutes to complete the enabling process.

Once it’s done ✅, you should be able to see that the composite index is enabled by going to Firebase Console Dashboard -> Database -> Indexes Tab

OrderBy and Limit Filters

Let’s take a look at orderBy() method which takes two arguments, the first one is a field name, the second one is optional and it defaults to ascending order (asc).

Let’s combine where and orderBy() by retrieving documents where age is greater than or equal to 20 and organize documents in descending order by age .

db.collection("users") .where("age", ">=", 20) .orderBy("age", "desc") .get() .then(snap => { snap.forEach(doc => { console.log(doc.data()); }); });

Nice!

Let’s add one more query filter called limit() .

By using the limit() method, you can restrict the number of documents count by passing an integer value.

db.collection("users") .where("age", ">=", 20) .orderBy("age", "desc") .limit(2) .get() .then(snap => { snap.forEach(doc => { console.log(doc.data()); }); });

That was easy!

Collection Group Queries

Collection Group Queries allow you to query data from different sub-collections that share the same name across the Firestore Database. It’s a new feature at the time the post was released at the Google I/O Conference 2019.

As I have shown earlier, I have a collection called users which have sub-collections called posts .

users/{userID}/posts/{postID}

Let’s say I want to get all the documents that were published in 2018 from posts sub-collections across the Firestore Database.

db.collectionGroup("posts") .where("publishedAt", ">=", new Date("2018-01-01 00:00")) .where("publishedAt", "<=", new Date("2018-12-31 23:59")) .get() .then(snap => { snap.forEach(doc => { console.log(doc.data()); }); });

Sub-collections that share the same name can be targeted using collectionGroup() instead of collection() .

When you want to compare two dates, you will need to use new Date() and pass the date that you want to check against the publishedAt on the posts sub-collection.

If you get a Missing or Insufficient Permission error on the browser / app debug console when you run the collection group query, the security rules need to be changed.

To change the Security Rules, Go to Firebase Console → Database→ Rules Tab and add the following code.

service cloud.firestore { match /databases/{database}/documents { match /{document=**} { allow read, write; } } }

Adding this code will allow users to read or write data to the Firestore database.

For demo purposes, I am allowing anyone to read or write data to the Firestore Database, but it’s not recommended. In a real-world scenario, you would have security rules targeting each collection or sub-collection to have different permissions based on your application behaviour.

Let’s run the collection group query one more time.

And, you will get another error on the browser/app debug console saying that the query requires a composite index.

This can be easily fixed by clicking the link provided by Firebase on the Debug console, which will then take you to the Firebase Dashboard Database Page.

Several minutes after, the composite index has been enabled.

Go back and try running the code one last time, and you will be able to see the results on the debug console.

Nice!

Firestore Pagination Queries using Query Cursors

Imagine, you have a huge collection of documents about cities. It’s obvious to split them into batches (pagination) using query cursor methods so that users can have more control on the data consumption.

This will prevent users from getting overcharged for their data plans. As a developer, Firebase will bill you based on the number of document reads.

Let’ see how to paginate the cities collection, showing the first 10 items when the next button is pressed, then the next 10, and so on as shown below.

I already have a cities collection imported into the Firestore Database.

There are only four query cursor functions. These functions will work with orderBy() by passing the document field that you want to paginate based off of.

startAt(): You can pass any field value from documents which will be the starting point of your query including that field value.

You can pass any field value from documents which will be the starting point of your query including that field value. startAfter() : This is very similar to the startAt() with the only difference being the value that you passed inside startAfter() is excluded in the result dataset.

: This is very similar to the startAt() with the only difference being the value that you passed inside startAfter() is excluded in the result dataset. endAt(): You can pass any field value from documents which will be the ending point of your query including that field value.

You can pass any field value from documents which will be the ending point of your query including that field value. endAfter(): This is very similar to the endAt() and the only difference is the value that you passed inside endAfter() is excluded in the result dataset.

First, I am going to create three global variables.

var cities = []; const cityRef = firebase.firestore().collection("cities") var lastVisibleCitySnapShot = {};

Then, declare and invoke a function called getFirstTenCities() which will get the first 10 cities from the Firestore Database.

const query = await this.cityRef.orderBy("city").limit(10); query.get().then(snap => { snap.forEach(doc => { this.cities.push(doc.data()); }); this.lastVisibleCitySnapShot = snap.docs[snap.docs.length - 1]; });

The initial query ordered by city field and limit to 10 items. Then, loop through the snap object that you receive from the call back function.

Append each city document to the cities global array that I declared earlier. After that, you can easily loop through the cities array in your HTML.

Finally, get the last document from the snap object using snap.docs array and assign it to lastVisibleCitySnapShot global variable.

I can get the next 10 items starting after the last city document stored in the lastVisibleCitySnapShot.

After that, add two simple next and previous buttons in your HTML and give a click event to them.

When a user clicks the next or next button, it invokes a function called next() .

async next() { this.cities = []; const query = await this.cityRef .orderBy("city") .startAfter(this.lastVisibleCitySnapShot) .limit(10); query.get().then(snap => { snap.forEach(doc => { this.cities.push(doc.data()); }); this.lastVisibleCitySnapShot = snap.docs[snap.docs.length - 1]; }); }

If you want to keep adding data to the cities array, keep scrolling down to see more data, you do not have reset it to an empty array [].

In my case, I want to reset it so that I can replace it with 10 new items. In the query, startAfter() method is used by passing the lastVisibleCitySnapshot . This will make sure to get only the next 10 items.

Similar to next() method, here is the prev() one.

async prev() { this.cities = []; const query = await this.cityRef .orderBy("city") .endBefore(this.lastVisibleCitySnapShot) .limit(10); query.get().then(snap => { snap.forEach(doc => { this.cities.push(doc.data()); }); this.lastVisibleCitySnapShot = snap.docs[snap.docs.length - 1]; }); }, ﻿

There you go.

Conclusion

You have learned how to do a simple CRUD operation type queries in Firestore. I have shown you how to filter data using WHERE, ORDERBY and LIMIT.

I have also taught you how to use Collection Group Queries and Pagination using Query Cursor Functions.

Now, I have a question for you…

What other queries would you like to know about which is not covered here?

Let me know in the comments section below so that I can add it here.

NEXT → Firebase Real-Time Database Querying, Sorting and Filtering