lb4 relations referential integrity for hasMany/belongsTo

[virkt25]

Description / Steps to reproduce / Feature proposal

As I was working on the user order history component for https://github.com/strongloop/loopback4-example-shopping I ran into an issue I thought LoopBack would've taken care of for me -- referential integrity with the @hasMany relation.

Current Behavior

I am able to create an order using /post/{userId}/orders for a userId which is not that of a user. I would've expected some sort of a check that would've ensure that the userId exists.

Expected Behavior

Throw an error for an userId not in the database.

Acceptance Criteria

For longer term, we can look into a better way to handle referential integrity. For now:

• check the database whether the userId exist and then make the post call

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[dhmlau]

In discussion with @raymondfeng @bajtos:
Proposed solution: check the database whether the userId exist and then make the post call
In the future, we can look into a better way to handle it.

[jannyHou]

Defer the estimation until we figure out at which level are we going to apply the constraint.
There are different approaches:

• repository level: the framework handles it for users, enforce on the database side
• controller level: fix the shopping example, enforce on the client side