Database
[{"_id":ObjectId("63c022bfd2d744ebaffe3412"),"buttons":[],"admin":[ObjectId("636bc633ccc1e71aa32ad831")],"user":ObjectId("636bc633ccc1e71aa32ad831"),"channelId":"1",}]
Query
db.collection.update({admin:ObjectId("636bc633ccc1e71aa32ad831"),channelId:"1"},[{$set:{admin:{$setUnion:["$admin",[ObjectId("63c022bfd2d744ebaffe341a")]]}}}])