Database
[{"_id":ObjectId("5a934e000102030405000009"),"followers":[ObjectId("5a934e0001020304050000f1"),ObjectId("5a934e0001020304050000f2"),ObjectId("5a934e0001020304050000f3")],"requests":[ObjectId("5a934e0001020304050000b1"),ObjectId("5a934e0001020304050000b2"),ObjectId("5a934e0001020304050000b3")]}]
Query
db.collection.update({_id:ObjectId("5a934e000102030405000009")},[{$set:{followers:{$concatArrays:["$followers","$requests"]}}},{$set:{requests:[]}}])