Database
[{"_id":ObjectId("60913f55987438922d5f0db6"),"procedureCode":"code1","description":"Description 1","coding":[{"system":"ABC","code":"L111"},{"system":"DEFG","code":"S222"}]},{"_id":ObjectId("60913f55987438922d5f0dbc"),"procedureCode":"code2","description":"Description 2","coding":[{"system":"ABC","code":"L999"},{"system":"DEFG","code":"X3333"}]}]
Query
db.collection.aggregate([{$match:{"coding.system":"ABC"}},{$unwind:"$coding"},{$match:{"coding.system":"ABC"}},{$group:{_id:null,coding:{$push:"$coding.code"}}}])