arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a[*]" ] }); { "deduplicate" : true, "fields" : [ "a[*]" ], "id" : "test/17920", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : false, "code" : 201 } arangosh> db.test.save({ a : [ 1, 2 ] }); { "_id" : "test/17923", "_key" : "17923", "_rev" : "_WnWX-Eu--_" } arangosh> db.test.save({ a : [ 1, 3 ] }); { "_id" : "test/17927", "_key" : "17927", "_rev" : "_WnWX-Eu--B" } arangosh> db.test.save({ a : null }); { "_id" : "test/17930", "_key" : "17930", "_rev" : "_WnWX-Ey--_" }