mongodb changestream“管道”不起作用

Question

I have some code that watches a mongodb collection for updates... I have it setup so that when anyone sends a message, the changestream will detect that... The issue is that when I try to add a pipeline, the updates do not get detected...

Here are some things that I've tried:

const pipeline = [
{ $match: { 'fullDocument.username_pair': 'HardCodedUsernameInDatabase' }}
];
changeStream = message_collection.watch(pipeline);

and

const pipeline = [
{ $match: { username_pair: 'HardCodedUsernameInDatabase' }}
];
changeStream = message_collection.watch(pipeline);

again, the code will detect all messages in the absence of any pipeline, ie:

changeStream = message_collection.watch();

...

UPDATE:

The reason why this is tricky is because the document I'm listening to looks like this:

_id:some_id

username_pair:"Test1234test1111"

messages:Array

The username_pair never updates, the messages array is what updates... I need to be looking at the document matching the username_pair, and I need to detect changes in the messages array that corresponds to the username_pair.

Answer 1

I guess the trick is to add option called fullDocument while opening the change stream on collection. see here

someCollection.watch([{
  "$match": {
    "operationType": {
       "$in": ["update"]
     },
     "fullDocument.username_pair": "HardCodedUsernameInDatabase"
  }
}], {"fullDocument": "updateLookup"})

Hope this helps