Don't project `_id` field in Mongo DB connector when it's not required

[krvikash]

Currently, _id field is retrieved even when it's not necessary. We can exclude by https://www.mongodb.com/docs/drivers/java/sync/current/fundamentals/builders/projections/#exclusion-of-_id.

Reference to #17710 (comment)

[Praveen2112]

We can also use projection(fields(include(fieldName)) - projection, fields include and exclude to handle them.

[sahoss]

@krvikash @Praveen2112 i'd like to take up this issue as its marked a good first issue.

[krvikash]

Thanks @sahoss for your interest to work on this. I have assigned this issue to you.

[sahoss]

@krvikash so from my understanding so far:

we need to make a change in mongdb/MongoSession#execute()
The change itself would be to check if Document output/List<MongoColumnHandle> columns contains _id. if it does, we dont exclude it. if it does not contain _id, then we follow the guidlines in the linked mongodb documention to exclude _id from projection.

I tried a few queries such as

select _id from orders, select * from customer inner join orders on customer._id = orders._id etc. to check if _id is populated by the caller in the argument to execute(List<MongoColumnHandle> columns) and it seems to be so. So for this issue, the outlined approach should work.

Also any way to test/assert what parameters were actually sent down to mongo? (without mocking, and mocking seems to discouraged in Development.md)

Can you please let me know your thoughts?

[krvikash]

Hi @sahoss, The approach looks fine. However, For testing, I am not really sure. But, we have to verify that MongoSession#execute should return the MongoCursor which does not have _id if not queried explicitly.