Database
db={events:[{_id:"38e88a8c8323...",items:[{ObjectID:"5e0f39e0d90a09c..."},{ObjectID:"6e0f39e0d90a09c..."}],other:"key"}],items:[{_id:"5e0f39e0d90a09c...",name:"Example"},{_id:"6e0f39e0d90a09c...",name:"Example"}]}
Query
db.events.aggregate([{$unwind:"$items"},{$lookup:{from:"items",localField:"items.ObjectID",foreignField:"_id",as:"items"}},{$group:{_id:"$_id",other:{$first:"$other"},items:{$push:{$arrayElemAt:["$items",0]}}}}])