Database
[{_id:1,type:"add",metadata:{schemes:{_trail:"ABCDEFG"}}},{_id:2,type:"edit",metadata:{schemes:[{_trail:"HIJKLMN"},{_trail:"OPQRSTU"}]}}]
Query
db.collection.aggregate([{$match:{"type":{$in:["add","edit"]},}},{$unwind:"$metadata.schemes"},{"$addFields":{"trail_beginning":{$substr:["$metadata.schemes._trail",0,3]}}},{$group:{"_id":{"trail_beginning":"$trail_beginning"},"count":{$sum:1},"items":{$push:"$$ROOT"},}},{"$sort":{count:-1}}])