Can query node as child but not as parent

I am trying to query all merchants who do not have a master vertex in Dgraph. I have a vertex that looks like this:

merchant: { id: "3", master: { id: "4", "dgraph.type": "Master" } }

Here is my query:

{ vertices(func:type(Merchant)) { id master { id } } }

This query will return all merchants, including those who have a master vertex. I want to modify this query to only return merchants who do not have a master vertex. Can anyone help me with this?

This should work:
{ vertices(func:type(Merchant)) @filter(not has(master)) { id master { id } } }

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