Monitor Data Changes

Sample Data

The examples in this guide use the sample_restaurants.restaurants collection from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see the Quick Start.

The examples on this page use the following Restaurant, Address, and GradeEntry classes as models:

public class Restaurant
{
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    [BsonElement("restaurant_id")]
    public string RestaurantId { get; set; }
    public string Cuisine { get; set; }
    public Address Address { get; set; }
    public string Borough { get; set; }
    public List<GradeEntry> Grades { get; set; }
}

public class Address
{
    public string Building { get; set; }
    [BsonElement("coord")]
    public double[] Coordinates { get; set; }
    public string Street { get; set; }
    [BsonElement("zipcode")]
    public string ZipCode { get; set; }
}

public class GradeEntry
{
    public DateTime Date { get; set; }
    public string Grade { get; set; }
    public float? Score { get; set; }
}

Include Pre-Images and Post-Images

By default, when you perform an operation on a collection, the corresponding change event includes only the delta of the fields modified by that operation. To see the full document before or after a change, create a ChangeStreamOptions object and specify the FullDocumentBeforeChange or the FullDocument options. Then, pass the ChangeStreamOptions object to the Watch() or WatchAsync() method.

The pre-image is the full version of a document before a change. To include the pre-image in the change stream event, set the FullDocumentBeforeChange option to one of the following values:

• ChangeStreamFullDocumentBeforeChangeOption.WhenAvailable: The change event includes a pre-image of the modified document for change events only if the pre-image is available.

• ChangeStreamFullDocumentBeforeChangeOption.Required: The change event includes a pre-image of the modified document for change events. If the pre-image is not available, the driver raises an error.

The post-image is the full version of a document after a change. To include the post-image in the change stream event, set the FullDocument option to one of the following values:

• ChangeStreamFullDocumentOption.UpdateLookup: The change event includes a copy of the entire changed document from some time after the change.

• ChangeStreamFullDocumentOption.WhenAvailable: The change event includes a post-image of the modified document for change events only if the post-image is available.

• ChangeStreamFullDocumentOption.Required: The change event includes a post-image of the modified document for change events. If the post-image is not available, the driver raises an error.

The following example opens a change stream on a collection and includes the post-image of updated documents by specifying the FullDocument option. Select the Asynchronous or Synchronous tab to see the corresponding code.

var pipeline = new EmptyPipelineDefinition<ChangeStreamDocument<Restaurant>>()
    .Match(change => change.OperationType == ChangeStreamOperationType.Update);
var options = new ChangeStreamOptions
{
    FullDocument = ChangeStreamFullDocumentOption.UpdateLookup,
};
using var cursor = await _restaurantsCollection.WatchAsync(pipeline, options);
await cursor.ForEachAsync(change =>
{
    Console.WriteLine(change.FullDocument.ToBsonDocument());
});

var pipeline = new EmptyPipelineDefinition<ChangeStreamDocument<Restaurant>>()
    .Match(change => change.OperationType == ChangeStreamOperationType.Update);
var options = new ChangeStreamOptions
{
    FullDocument = ChangeStreamFullDocumentOption.UpdateLookup,
};
using (var cursor = _restaurantsCollection.Watch(pipeline, options))
{
    foreach (var change in cursor.ToEnumerable())
    {
        Console.WriteLine(change.FullDocument.ToBsonDocument());
    }
}

Running the preceding code example and updating a document that has a "name" value of "Blarney Castle" results in the following change stream output:

{ "_id" : ObjectId("..."), "name" : "Blarney Castle", "restaurant_id" : "40366356",
"cuisine" : "Traditional Irish", "address" : { "building" : "202-24", "coord" : [-73.925044200000002, 40.5595462],
"street" : "Rockaway Point Boulevard", "zipcode" : "11697" }, "borough" : "Queens", "grades" : [...] }

To learn more about pre-images and post-images, see Change Streams with Document Pre- and Post-Images in the MongoDB Server manual.

API Documentation

To learn more about any of the methods or types discussed in this guide, see the following API documentation:

• Watch()

• WatchAsync()

• ChangeStreamOptions

• UpdateOne()