How do I get neo4j nodes based on schema index?

Question

I am using the ruby gem neography

I created a schema index based on a label Client and a key name client_name

I would want to get the node for client_name => "some name"

How can I get the node based on the schema index and the client_name as key. Not to search which can return more than one, but to specify the key/value pair as above?