Generating mongodb update query dynamically in Spring Data

Issue

I am currently implementing a Mongo ChangeStream poller in which I would like to monitor a collection for any changes. I was originally intending to write the updates to a change log and trigger some other actions. However, I found out that the save() call in Spring Data performs a document replace instead of update. Therefore, I only have access to the new document instead of the delta as would occur during an update.

There are two options I see:

1. Replace the save call with something like updateOne
2. Update the collection with changeStreamPreAndPostImages set to true and diff the two documents

If I go with option 1, I'm wondering if there is an easy way to generate the update query dynamically based on the existing document and the updated document? Prior to the save/update I fetch the current document and update the values before saving again. I couldn't find any built in support. I'm guessing I'd have to take a reflection based approach, but I wanted to see if there's any existing solutions already.

Solution

Ultimately, I went with option 2 as it turned out to be quite easy to enable changeStreamPreAndPostImages on application start up. I created an extension on the mongo client:

fun MongoDatabase.enablePreImageChangeStream(collection: String) {
    this.runCommand(
        BsonDocument.parse(
            """
               {
                   "collMod": "$collection",
                   "changeStreamPreAndPostImages": { "enabled": true }
               }
            """
        )
    )
}

Then on application start up I did:

@Configuration
class ChangeStreamConfiguration(
    private val mongoTemplate: MongoTemplate,
) {
    init {
        mongoTemplate.db.enablePreImageChangeStream("<collection name here>")

  }
}

Answered By - fbailey