What you are trying to do? Can't you just dissoc from final result?

A mutation in the db, we want to actually dissoc from the data at rest

and we can’t depend the absence of a key corresponding to an implied dissoc in the case of an update because partial updates are possible

so client sends up (entity/update {:e/uuid "XXX" :e/joined-obj :pathom.ops/dissoc})

and it would remove the :e/joined-obj key from the object at rest