db={orders:[{_id:1,tasks:[{_id:1,name:"task1"},{_id:2,name:"task2"}]}],taskinvites:[{_id:1,status:"ACCEPTED",task:1,name:"Invite one"}]}

db.orders.aggregate([{$match:{_id:1}},{$unwind:{path:"$tasks",preserveNullAndEmptyArrays:true}},{$lookup:{from:"taskinvites",let:{taskId:"$tasks._id"},pipeline:[{$match:{$expr:{$and:[{$in:["$status",["ACCEPTED","UNDER_REVIEW"]]},{$eq:["$task","$$taskId"]}]}}}],as:"invite"}}])