arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a" ] }); { "id" : "test/15125", "type" : "hash", "fields" : [ "a" ], "selectivityEstimate" : 1, "unique" : false, "sparse" : false, "isNewlyCreated" : true, "code" : 201 } arangosh> db.test.save({ a : 1 }); { "_id" : "test/15128", "_key" : "15128", "_rev" : "_UWy-Mqu---" } arangosh> db.test.save({ a : 1 }); { "_id" : "test/15132", "_key" : "15132", "_rev" : "_UWy-Mqu--_" } arangosh> db.test.save({ a : null }); { "_id" : "test/15135", "_key" : "15135", "_rev" : "_UWy-Mqy---" }