Relation repositories don't have direct access to foreign key value

[b-admike]

_Originally posted by @bajtos in https://github.com/strongloop/loopback-next/pull/2005/files#r237517294

HasOneRepository and HasManyRepository don't have direct access to the foreign key value to throw meaningful errors (e.g. EntityNotFoundError) with it. Currently get() and find() for these repositories could benefit from having access to it, but there are other methods as well.

[dhmlau]

@b-admike, is this still needed? If yes, could you please add the acceptance criteria? Thanks.

[bajtos]

Here is the issue: when a relation method is trying to access a related model instance and no instance with the foreign key was found, we want to throw a helpful EntityNotFoundError. The constructor of EntityNotFoundError requires entity id as an argument, the code in relation repositories does not have the id at hand, only a constraint like {[foreignKey]: value}. It would be great to find a better solution. For example, we can extend EntityNotFoundError to accept either id (as now) or a constraint (a where filter) too.

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[stale]

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