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How to Use Firestore Increment
2019-04-08 07:41:02 -0700
2019-04-08 07:41:02 -0700
Jeff Delaney
Use the special Firestore Increment field value to increase or decrease numeric values atomically in the database.

One of the common challenges faced when working with Firestore is maintaining an accurate count of a value on a document (or multiple documents). One cannot simply update a counter and expect it to be accurate because many clients might be competing to update the same value simultaneously. The solution to this problem has traditionally been to deploy a Cloud Function that runs the update in a transaction to guarantee an atomic update. This works great, but adds a significant amount of complexity for a relatively simple requirement.

Fortunately, Firebase recently introduced FieldValue.increment to make this process much, much easier. Let's take a look at how it works...

Firestore Increment

Increment is a special value that will atomically update a counter based on the interval you specify. You do not need to read the document to obtain the current total - Firebase does all the calculation serverside. In most cases, you will be keeping a count by adding an integer of 1 to the current total.

Increase a Counter

{{< file "js" "app.js" >}} {{< highlight javascript >}} const db = firebase.firestore(); const increment = firebase.firestore.FieldValue.increment(1);

// Document reference const storyRef = db.collection('stories').doc('hello-world');

// Update read count storyRef.update({ reads: increment }); {{< /highlight >}}

Decrease a Counter

We can decrement a counter by simply changing the interval value to -1.

{{< file "js" "app.js" >}} {{< highlight javascript >}} const decrement = firebase.firestore.FieldValue.increment(-1);

// Update read count storyRef.update({ reads: decrement }); {{< /highlight >}}

Update a Floating Point Total

It also works with decimal values, which can be useful when updating something like a user's order history total.

{{< highlight javascript >}} const userRef = db.collection('users').doc('some-user'); const increaseBy = firebase.firestore.FieldValue.increment(23.99);

userRef.update({ totalSpent: increaseBy }); {{< /highlight >}}

Counting Values in Separate Documents

In certain cases, your counter value may depend on the creation of a new document or just be located in a different document. You can use batched writes to ensure all data is updated safely. For example, we might want to create a like document, then update the total multiple places.

{{< highlight javascript >}} const batch = db.batch(); batch.set(likesRef, ...likeData); batch.update(storyRef, { likes: increment }); batch.update(userRef, { totalLikes: increment }); batch.commit(); {{< /highlight >}}

Keep a Count of Documents in a Collection

Keeping a count of documents in a collection is perhaps the single most common form of data aggregation in Firestore. Prior to increment, we had the option to (a) read ALL documents clientside or (b) use a Cloud Function to keep a count of documents in a collection.

Now, we have a third option. Create a document called --stats-- for the purpose of maintaining aggregation or metadata about this collection. In your frontend code, use a batch to increment the stats when creating a new document.

{{< highlight javascript >}} const storyRef = db.collection('stories').doc('hello-world'); const statsRef = db.collection('stories').doc('--stats--');

const increment = firebase.firestore.FieldValue.increment(1);

const batch = db.batch(); const storyRef = db.collection('stories').doc(${Math.random()}); batch.set(storyRef, { title: 'New Story!' }); batch.set(statsRef, { storyCount: increment }, { merge: true }); batch.commit(); {{< /highlight >}}

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