MongoDB not emitting `before` for DELETE events

[Robin Tang]

Hey team,

As the subject suggests, the MongoDB connector is not emitting a `before` object for deletes.

Version: Debezium 2.5

MongoDB: 5.0.14

My Debezium config (using Connect)

```

{
    "name": "mongodb-connector",
    "config": {
        "connector.class" : "io.debezium.connector.mongodb.MongoDbConnector",
        "tasks.max" : "1",
        "mongodb.connection.string" : "mongodb://mongodb:27017/?replicaSet=rs0",
        "topic.prefix" : "dbserver1",
        "mongodb.user" : "debezium",
        "mongodb.password" : "dbz",
        "mongodb.capture.mode": "change_streams_update_full_with_pre_image"
    }
}

```

^ I don't think capture.mode makes a difference here since this pertains to updates only, but added anyways. 

Using the example:

```

rs0:PRIMARY> db.customers.remove({"_id": 1001});
WriteResult({ "nRemoved" : 1 })
rs0:PRIMARY>

```

```

$ kafka-console-consumer --topic dbserver1.inventory.customers --bootstrap-server localhost:29092
{"schema":{"type":"struct","fields":[{"type":"string","optional":true,"name":"io.debezium.data.Json","version":1,"field":"before"},{"type":"string","optional":true,"name":"io.debezium.data.Json","version":1,"field":"after"},{"type":"struct","fields":[{"type":"array","items":{"type":"string","optional":false},"optional":true,"field":"removedFields"},{"type":"string","optional":true,"name":"io.debezium.data.Json","version":1,"field":"updatedFields"},{"type":"array","items":{"type":"struct","fields":[{"type":"string","optional":false,"field":"field"},{"type":"int32","optional":false,"field":"size"}],"optional":false,"name":"io.debezium.connector.mongodb.changestream.truncatedarray","version":1},"optional":true,"field":"truncatedArrays"}],"optional":true,"name":"io.debezium.connector.mongodb.changestream.updatedescription","version":1,"field":"updateDescription"},{"type":"struct","fields":[{"type":"string","optional":false,"field":"version"},{"type":"string","optional":false,"field":"connector"},{"type":"string","optional":false,"field":"name"},{"type":"int64","optional":false,"field":"ts_ms"},{"type":"string","optional":true,"name":"io.debezium.data.Enum","version":1,"parameters":{"allowed":"true,last,false,incremental"},"default":"false","field":"snapshot"},{"type":"string","optional":false,"field":"db"},{"type":"string","optional":true,"field":"sequence"},{"type":"string","optional":false,"field":"rs"},{"type":"string","optional":false,"field":"collection"},{"type":"int32","optional":false,"field":"ord"},{"type":"string","optional":true,"field":"lsid"},{"type":"int64","optional":true,"field":"txnNumber"},{"type":"int64","optional":true,"field":"wallTime"}],"optional":false,"name":"io.debezium.connector.mongo.Source","field":"source"},{"type":"string","optional":true,"field":"op"},{"type":"int64","optional":true,"field":"ts_ms"},{"type":"struct","fields":[{"type":"string","optional":false,"field":"id"},{"type":"int64","optional":false,"field":"total_order"},{"type":"int64","optional":false,"field":"data_collection_order"}],"optional":true,"name":"event.block","version":1,"field":"transaction"}],"optional":false,"name":"dbserver1.inventory.customers.Envelope"},"payload":{"before":null,"after":null,"updateDescription":null,"source":{"version":"2.5.3.Final","connector":"mongodb","name":"dbserver1","ts_ms":1711305148000,"snapshot":"false","db":"inventory","sequence":null,"rs":"rs0","collection":"customers","ord":1,"lsid":null,"txnNumber":null,"wallTime":null},"op":"d","ts_ms":1711305148854,"transaction":null}}

```

Thanks!

Robin

[Robin Tang]

Just want to bump this one up. I think the example listed here: https://debezium.io/documentation/reference/stable/connectors/mongodb.html#mongodb-delete-events is wrong as delete never emits `before`.

- Robin

[Jakub Čecháček]

Hi Robin,

the before field is populated from the fullDocumentBeforeChange filed which comes from Mongo's change event document received by Debezium from the database. The presence of this field requires that before image are enabled on database level for the watched collection. Please see the following references 

https://www.mongodb.com/docs/manual/changeStreams/#change-streams-with-document-pre--and-post-images

https://www.mongodb.com/docs/manual/reference/command/collMod/#change-streams-with-document-pre--and-post-images

Regards,

Jakub.