db={"matches":[{date:"2020-02-15T17:00:00Z",players:[{_id:"5efd9485aba4e3d01942a2ce"},{_id:"5efd9485aba4e3d01942a2cf"}]},],"players":[{_id:"5efd9485aba4e3d01942a2ce",name:"Rafa Nadal"},{_id:"5efd9485aba4e3d01942a2cf",name:"Roger Federer"},]}

db.matches.aggregate([{$unwind:"$players"},{$lookup:{from:"players",let:{id:"$players._id"},pipeline:[{$match:{$expr:{$eq:["$_id","$$id"]}}}],as:"players"}}])