arango> db.four.ensureUniqueConstraint("a", "b.c");
{ "id" : "164405/1147445", "unique" : true, "type" : "hash", "fields" : ["a", "b.c"], "isNewlyCreated" : true }

arango> db.four.save({ a : 1, b : { c : 1 } });
{ "_id" : "164405/1868341", "_rev" : 1868341 }

arango> db.four.save({ a : 1, b : { c : 1 } });
JavaScript exception in file '(arango)' at 1,9: [ArangoError 1210: cannot save document]
!db.four.save({ a : 1, b : { c : 1 } });
!        ^
stacktrace: [ArangoError 1210: cannot save document]
    at (arango):1:9

arango> db.four.save({ a : 1, b : { c : null } });
{ "_id" : "164405/2196021", "_rev" : 2196021 }

arango> db.four.save({ a : 1 });
{ "_id" : "164405/2196023", "_rev" : 2196023 }