[{title:"My book",authors:[{id:1,date:"2023-12-20"},{id:2,date:"2023-12-21"}],authorsDetails:[{id:2,name:"Author 2",age:25},{id:1,name:"Author 1",age:38}]}]

db.collection.aggregate([{$project:{title:1,authors:{$concatArrays:["$authors","$authorsDetails"]}}},{$unwind:"$authors"},{$group:{_id:{_id:"$authors.id",title:"$title"},authors:{$mergeObjects:"$authors"}}},{$group:{_id:"$_id.title",authors:{$push:"$authors"}}},{$set:{title:"$_id"}},{$unset:["_id","authors.age"]}])