Update the datasource/database read preference at the run time.

[shivambmgupta]

Describe the bug

For architectural reasons, I need to update the readPreference for a given method, on a given repository at the fly.
Please note, this change should be done at real time, and impact only this function invocation. However, the readPreferrence at the application level/datasource level should remain intact to "primaryPreferred".

For example,

this.userRepo.find(filter, { readPreference: "secondary" }); // request is still going to primary MongoDB server!
this.userRepo.findById(filter, { readPreference: "primary" });

I've tried this, but upon checking mongodb logs, this does not seem to work. All the request are going to primary server.

MongoDB version: v4.2

Logs

NA

Additional information

NA

Reproduction

https://github.com/shivambmgupta/loopback-next/tree/bug/read-preference-update

[achrinza]

Thanks for raising this issue, @shivambmgupta. I believe the MongoDB connector simply does not pass this option to the underlying driver.

I've not had time to review this more in-depth, but I'll see if I can timebox in the upcoming weeks.

If you'd like, you can also contribute a fix over at https://github.com/loopbackio/loopback-connector-mongodb.

v7 is currently in pre-release, so it should be easier to contribute the change.