arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a" ] }); { "deduplicate" : true, "fields" : [ "a" ], "id" : "test/16208", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : false, "code" : 201 } arangosh> db.test.save({ a : 1 }); { "_id" : "test/16211", "_key" : "16211", "_rev" : "_XiAwU6u--_" } arangosh> db.test.save({ a : 1 }); { "_id" : "test/16215", "_key" : "16215", "_rev" : "_XiAwU6u--B" } arangosh> db.test.save({ a : null }); { "_id" : "test/16218", "_key" : "16218", "_rev" : "_XiAwU6y--_" }