Our food co-op site uses relationships, e.g. Household Member. We have a custom relationship type: **Proxy Shopper**. We're interested in making Proxy Shopper conditional so that no individual can get Proxy Shopper status **unless** both individuals are Members of the same Household.

> 
> *Example:*
> 
> 
> Jerry and Willow are Members of the Ortega Household. Willow
>  owns a co-op membership with which she can shop at our store, and
>  Jerry does not own such a membership. However, Willow has designated
>  Jerry as a Proxy Shopper so he'll be able to shop on her behalf.
> 
> 
> Willow's mother Sabrina, however, is a member of the Sturm household
>  (not Ortega), so Sabrina should not be a proxy shopper for Willow,
>  even though Willow wants to send her mother to the co-op.
> 
> 
> 

In short, **we're looking for a way (possibly through an extension) to condition the Proxy Shopper relationship type upon membership in the same Household**.

Worth noting that while belonging to the same Household should be required for setting a Proxy Shopper relationship, Household membership should not predict a Proxy Shopper relationship. In other words, no Household Member should become a Proxy Shopper unless someone sets that relationship via the admin interface.

What's the best way to do this -- beyond just telling backend users that they shouldn't make anyone a Proxy Shopper unless that individual belongs to the same Household as the second individual?