arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a[*]" ] }); { "deduplicate" : true, "fields" : [ "a[*]" ], "id" : "test/110999", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : false, "code" : 201 } arangosh> db.test.save({ a : [ 1, 2 ] }); { "_id" : "test/111002", "_key" : "111002", "_rev" : "_YOn1f1y--_" } arangosh> db.test.save({ a : [ 1, 3 ] }); { "_id" : "test/111006", "_key" : "111006", "_rev" : "_YOn1f12--_" } arangosh> db.test.save({ a : null }); { "_id" : "test/111009", "_key" : "111009", "_rev" : "_YOn1f12--B" }