Provider SDK - Enforce uniqueness of a resource

I am developing a provider and have 2 resources A and B , we are allowing to create as any A resources as you wish. A B resource gets as input the ID of the A resources and there can be only 1 for every A.

I don’t seem to find anything in the SDK to enforce that , and i cant really query during the creation time and see whether something like that does already exists in our API or not