Database Triggers

On this page

  • Overview
  • Example
  • Configuration
  • Database Change Events
  • Suspended Triggers

Database Triggers allow you to execute server-side logic whenever a document is added, updated, or removed in a linked MongoDB Atlas cluster. You can use database Triggers to implement complex data interactions, including updating information in one document when a related document changes or interacting with a service upon the insertion of a new document.

Database Triggers use MongoDB change streams to listen for changes to documents in a collection and pass database events to their associated Trigger function.

Change Stream Limitations

MongoDB Realm opens a single change stream for each collection with at least one enabled Trigger and limits the total number of open change streams on each linked cluster across all Realm apps based on the cluster's size. See change stream limitations for more information.


Database Triggers are only available for MongoDB Atlas clusters that are running MongoDB version 3.6 or newer.

An online store wants to notify its customers whenever one of their orders changes location. They record each order in the store.orders collection as a document that resembles the following:

_id: ObjectId("59cf1860a95168b8f685e378"),
customerId: ObjectId("59cf17e1a95168b8f685e377"),
orderDate: ISODate("2018-06-26T16:20:42.313Z"),
shipDate: ISODate("2018-06-27T08:20:23.311Z"),
orderContents: [
{ qty: 1, name: "Earl Grey Tea Bags - 100ct", price: NumberDecimal("10.99") }
shippingLocation: [
{ location: "Memphis", time: ISODate("2018-06-27T18:22:33.243Z") },

To automate this process, the store creates a database Trigger that listens for UPDATE change events in the store.orders collection. When the trigger observes an UPDATE event, it passes the change event object to its associated Function, textShippingUpdate. The Function checks the change event for any changes to the shippingLocation field and, if it was updated, sends a text message to the customer with the new location of the order.

exports = async function (changeEvent) {
// Destructure out fields from the change stream event object
const { updateDescription, fullDocument } = changeEvent;
// Check if the shippingLocation field was updated
const updatedFields = Object.keys(updateDescription.updatedFields);
const isNewLocation = updatedFields.some(field =>
// If the location changed, text the customer the updated location.
if (isNewLocation) {
const { customerId, shippingLocation } = fullDocument;
const twilio ="myTwilioService");
const mongodb ="mongodb-atlas");
const customers = mongodb.db("store").collection("customers");
const { location } = shippingLocation.pop();
const customer = await customers.findOne({ _id: customer_id })
to: customer.phoneNumber,
from: context.values.get("ourPhoneNumber"),
body: `Your order has moved! The new location is ${location}.`

Database Triggers have the following configuration options:

Trigger Type
Required. The type of the Trigger. Set this value to DATABASE for database Triggers
Trigger Name
Required. The name of the Trigger.
Linked Function
Required. The name of the Realm Function that the Trigger executes whenever it fires. The Trigger passes the database event object that caused it to fire as the only argument to this Function.
Required. The name of the MongoDB Service that the Trigger is associated with.
Database Name
Required. The MongoDB database that contains the watched collection.
Collection Name
Required. The name of the collection that the Trigger watches for change events.
Operation Types

Required. A list of one or more database operation types that cause the Trigger to fire. Format each operation type as an uppercase string, e.g., "INSERT".


Update operations executed from MongoDB Compass or the MongoDB Atlas Data Explorer fully replace the previous document. As a result, update operations from these clients will generate REPLACE change events rather than UPDATE events.

Full Document

If enabled, UPDATE change events include the latest majority-committed version of the modified document after the change was applied in the fullDocument field.


Regardless of this setting:

  • INSERT and REPLACE events always include the fullDocument field.
  • DELETE events never include the fullDocument field.
Document Preimage

If enabled, change events include a copy of the modified document from immediately before the change was applied in the fullDocumentBeforeChange field. All change events except for INSERT events include the document preimage.

Collection-Level Preimage Settings

Document preimages use extra information stored in the oplog. The extra data may have performance implications for some apps.

Once you've enabled document preimages for any trigger on a given collection, that collection will include preimage data in the oplog and other triggers on the collection can use preimages with no additonal overhead.

You can disable document preimages on a per-trigger basis to exclude the preimage from change events. Regardless of your trigger-level settings, a collection's oplog entries will continue to include preimage data unless you explicitly disable preimages for the collection.

For more information, see View & Disable Collection-level Preimages.

Event Ordering

If event ordering is enabled, multiple executions of this Trigger will occur sequentially based on the timestamps of the change events. If event ordering is disabled, multiple executions of this Trigger will occur independently.


Consider disabling event ordering if your trigger fires on a collection that receives short bursts of events (e.g. inserting data as part of a daily batch job).

Ordered Triggers wait to execute a Function for a particular event until the Functions of previous events have finished executing. As a consequence, ordered Triggers are effectively rate-limited by the run time of each sequential Trigger function. This may cause a significant delay between the database event and the Trigger firing if a sufficiently large number of Trigger executions are currently in the queue.

Unordered Triggers execute functions in parallel if possible, which can be significantly faster (depending on your use case) but does not guarantee that multiple executions of a Trigger Function occur in event order.

Match Expression


A $match expression document that Realm uses to filter which change events cause the Trigger to fire. The Trigger evaluates all change event objects that it receives against this match expression and only executes if the expression evaluates to true for a given change event.

Use Dot-Notation for Embedded Fields

MongoDB performs a full equality match for embedded documents in a match expression. If you want to match a specific field in an embedded document, refer to the field directly using dot-notation. For more information, see Query on Embedded Documents in the MongoDB server manual.


The following Match Expression configures a trigger to fire only if the change event object specifies that the status field in a document changed.

"updateDescription.updatedFields.status": {
"$exists": true
Project Expression


A $project expression document that Realm uses to filter the fields that appear in change event objects.


A trigger is configured with the following Project Expression:

"_id": 0,
"operationType": 1,
"updateDescription.updatedFields.status": 1

The change event object that Realm passes to the trigger function only includes the fields specifed in the projection, as in the following example:

"operationType": "update",
"updateDescription": {
"updatedFields": {
"status": "InProgress"

Database change events represent individual changes in a specific collection of your linked MongoDB Atlas cluster.

Every database event has the same operation type and structure as the change event object that was emitted by the underlying change stream. Change events have the following operation types:

Operation Type
Represents a new document added to the collection.
Represents a change to an existing document in the collection.
Represents a new document that replaced a document in the collection.
Represents a document deleted from the collection.

Database change event objects have the following general form:

_id : <ObjectId>,
"operationType": <string>,
"fullDocument": <document>,
"fullDocumentBeforeChange": <document>,
"ns": {
"db" : <string>,
"coll" : <string>
"documentKey": {
"_id": <ObjectId>
"updateDescription": <document>,
"clusterTime": <Timestamp>

Database Triggers may enter a suspended state in response to an event that prevents the Trigger's change stream from continuing. Events that can suspend a Trigger include:

  • invalidate events such as dropDatabase, renameCollection, or those caused by a network disruption
  • the token required to resume the change stream is no longer in the cluster oplog
  • importing a Realm app with the replace strategy instead of the merge strategy and consequently removing the database user that created the Trigger's change stream

When resuming the suspended Trigger, Realm attempts to resume the Trigger at the next change stream event after the change stream stopped. However, if the resume token is no longer in the cluster oplog, the Trigger begins listening to new events but does not process any missed past events. You can adjust your oplog size to keep the resume token around for more time after a suspension.

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On this page

  • Overview
  • Example
  • Configuration
  • Database Change Events
  • Suspended Triggers