MongoDB：如何在对象数组中找到_id $？

Question

I have a "group" collection which groups documents from another "item" collection. The group collection is simply an array of ObjectId's from the item collection, however I want to make the objects in the group array more complex and still be able to use the $in and $nin operators.

How can I use $in when changing the value used with the $in operator to an object instead of only an ObjectId?

// current group document
{
  _id: ObjectId(...),
  items: [
    ObjectId(...),
    ObjectId(...)
  ]
}
// current item find query
db.item.find({ _id : { $in: group.items } }

I would like to change the group document to look like the following

{
  _id: ObjectId(...),
  items: [
     { _id : ObjectId(...), name: '...' },
     { _id : ObjectId(...), name: '...' }
  ]
}

Answer 1

Transform your group array to a simple array of ids to use for your find():

// revised item find logic
var itemGroupItemIds = group.items.map(function(item){
    return item._id
});
db.item.find({ _id : { $in: itemGroupItemIds } });