I think what you are seeing here is the limitation of GraphQL. If you study this rabbit trail a little bit more, you will find that what you want is supported 100% in DQL with var blocks. So why is it not supported yet in Dgraph’s generated GraphQL API—because the GraphQL spec does not support the idea of var blocks. hence one of the reasons that Dgraph had to come up with DQL in the first place. I wrote up an idea of how to get Dgraph to generate var blocks in DQL for a given GraphQL filter input. This does become a quite complex rewrite process though which stands to reason why it has not been built out yet.
I have done what you are asking for in pure GraphQL, but it involves at least 2 separate query operations. I will be demoing this on Dgraph Day.