arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a[*]" ] }); { "deduplicate" : true, "fields" : [ "a[*]" ], "id" : "test/16258", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : false, "code" : 201 } arangosh> db.test.save({ a : [ 1, 2 ] }); { "_id" : "test/16261", "_key" : "16261", "_rev" : "_W0jzAX6--_" } arangosh> db.test.save({ a : [ 1, 3 ] }); { "_id" : "test/16265", "_key" : "16265", "_rev" : "_W0jzAX6--B" } arangosh> db.test.save({ a : null }); { "_id" : "test/16268", "_key" : "16268", "_rev" : "_W0jzAX6--D" }