Database
[{"_id":ObjectId("5f56f8e66a7eee227c1acc0a"),"title":"Book1","author":"Author1","chapters":[{"_id":ObjectId("5f56fa47a78fbf03cc32d16d"),"title":"Chapter1","number":1},{"_id":ObjectId("5f56fad10820300de031317f"),"title":"Chapter2","number":2}]}]
Query
db.collection.aggregate([{$match:{"chapters._id":ObjectId("5f56fad10820300de031317f")}},{$replaceWith:{$arrayElemAt:[{$filter:{input:"$chapters",cond:{$eq:["$$this._id",ObjectId("5f56fad10820300de031317f")]}}},0]}}])