Database
db={"client":[{_id:1,taxID:"aldsfkjasdlñfk"}],"invoices":[{_id:1,clientID:1,total:50},{_id:2,clientID:1,total:150},{_id:3,clientID:1,total:200}]}
Query
db.client.aggregate([{$lookup:{from:"invoices",localField:"_id",foreignField:"clientID",as:"invoices"}},{$set:{"invoicesAmountGreaterThanOneHundred":{$reduce:{input:"$invoices",initialValue:"",in:{$sum:["$$value",{$cond:{if:{$gte:["$$this.total",100]},then:"$$this.total",else:0}}]}}}}}])