[Feature] Filter in level 2 properties

[Sturgelose]

Feature proposal

Summary: Allow to filter by properties in relations (level 2 properties).
Generally, this mostly is a JOIN in SQL language.

Until now there have been many issues in the strongloop/loopback and the strongloop/loopback-datasource-juggler repositories, but until now, none of them has been really been fixed. In the most discussed issue, which started in 2014 BTW (strongloop/loopback#517 (comment)) , it was said to please open an issue here, but nothing has been done yet.

As it seems the main problem is to include both a way to query to the API in a level 2 property, but also to create an API in juggler to handle this.

As I see, currently the initial discussions on how to implement Juggler have been held (#537 (comment)), but it's not clear in which point this kind of "features" should be started to be discussed or implemented. As this feature affects not only the internal Juggler logic, but also the API querying method, shouldn't this already taken into account in this stage?

Maybe, we could create an issue to gather all this relevant and "painful" issues, so they can be taken in account since the start. I'm not sure if this has already been done, but at least for someone who is following the development of loopback-next from the outside, I haven't been able to find anything related to this in all the issues of this repo.

Other examples of this kind of Juggler issues are:

• Bulk operations (I have seen this discussed as a major feature in loopback-next)
• MultiColumn primary keys: (they work, but you cannot find by ID or similars)
• More transparent migrations (this was also discussed in loopback-next Juggler threads)

Anyways, thanks a lot for the development happening here, and hoping to see it become reality soon!
As someone working with DBs, async/awaits and Observables daily, I see this project as "magic" that helps save a lot of time, so even if I have much limited time to help in this project, if there is any way to help in these issues with my knowledge, I'd be willing to help them to be implemented and fixed. Thanks again!

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This issue has been marked stale because it has not seen activity within six months. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository. This issue will be closed within 30 days of being stale.

[stale]

This issue has been closed due to continued inactivity. Thank you for your understanding. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository.