db={issues:[{project:ObjectId("5e3c27bf1ef77236945ef067"),/** ref to a project*/id:101,assignedTo:ObjectId("5e3c27bf1ef77236945e1212")/** ref to a user*/}],project:[{_id:ObjectId("5e3c27bf1ef77236945ef067"),title:"Proj 1",members:[{_id:ObjectId("5e3c27bf1ef7723694213213"),user:ObjectId("5e3c27bf1ef77236945e1212"),/** ref to a user*/discipline:"Architect"},{_id:ObjectId("5e3c27bf1ef772369421321a"),user:ObjectId("5e3c27bf1ef77236945e1217"),/** ref to a user*/discipline:"Architect"}]}]}

db.issues.aggregate([{$lookup:{from:"project",localField:"project",foreignField:"_id",as:"project",let:{assignedTo:"$assignedTo"},pipeline:[{$project:{members:{$filter:{input:"$members",cond:{$eq:["$$this.user","$$assignedTo"]}}}}}]}}])