arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a" ] }); { "id" : "test/15826", "type" : "hash", "fields" : [ "a" ], "selectivityEstimate" : 1, "unique" : false, "sparse" : false, "isNewlyCreated" : true, "code" : 201 } arangosh> db.test.save({ a : 1 }); { "_id" : "test/15829", "_key" : "15829", "_rev" : "15829" } arangosh> db.test.save({ a : 1 }); { "_id" : "test/15833", "_key" : "15833", "_rev" : "15833" } arangosh> db.test.save({ a : null }); { "_id" : "test/15836", "_key" : "15836", "_rev" : "15836" }