[{"key":1,"a":[5,10,25]},{"key":2,"a":[10,11,15]}]

db.collection.aggregate([{$addFields:{valid:{$size:{$filter:{input:"$a",as:"item",cond:{$and:[{$gte:["$$item",12]},{$lte:["$$item",17]}]}}}}}},{$match:{valid:{$gt:0}}},{$unset:"valid"}])