Make key required when a different key has specific value?

[klingelschmidt]

Is it possible to make an optional key required when a different key is set to a value?
I though it could maybe be like this:

"datasource":   jio.String().Valid("foo", "bar").Required(),

"id":           jio.Number().When("datasource", "bar", jio.Number().Integer().Required()),

But it did not seem to work.