我无法正确获得查询的答案 MongoDB - Find 方法

Question

My JSON:

  { 
    "user_id" : "1000" ,
    "boxes" : [
        {
            "box_id" : "12345", 
            "box_name" : "Box 3", 
            "items" : [], 
            "visible" : false 
        },
        {
            "box_id" : "2000", 
            "box_name" : "Box 1", 
            "items" : [],
            "visible" : true 
        }, 
        {
            "box_id" : "3000", 
            "box_name" : "Box 2", 
            "items" : [], 
            "visible" : true 
        }
    ], 
    "user_name" : "Jimmy", 
}

I'm just trying to get which box belongs to user_id:"1000" , which one's visible: true and which one's box_id: "3000" . My query is:

db.getCollection("users")
   .find({user_id:"1000", 
          $and: [{"boxes.box_id": "3000"},{"boxes.visible": true}]}, 
          {"boxes.$":1})

But I'm always getting box_id:12345 one. I couldn't find anything about it. Thank you for answers.

Answer 1

According to the documentation for the positional $ operator , you can't use the $ with multiple conditions. You have to use $elemMatch instead to work around the limitation.

Array Field Limitations

MongoDB requires the following when dealing with projection over arrays:

• Only one positional $ operator may appear in the projection document.
• Only one array field, the one being limited with the $ projection operator, should appear in the query document. Additional array fields in the query document may lead to undefined behavior.
• The query document should only contain a single condition on the array field being projected. Multiple conditions may override each other internally and lead to undefined behavior.

Under these requirements, the following query is incorrect:

 db.collection.find( { <array>: <value>, <someOtherArray>: <value2> }, { "<array>.$": 1 } )

To specify criteria on multiple fields of documents inside that array, use the $elemMatch query operator. The following query returns the first document inside a grades array that has a mean of greater than 70 and a grade of greater than 90.

 db.students.find( { grades: { $elemMatch: { mean: { $gt: 70 }, grade: { $gt:90 } } } }, { "grades.$": 1 } )

You must use the $elemMatch operator if you need separate conditions for selecting documents and for choosing fields within those documents.

So I would try a query like:

db.users.find({ user_id:"1000", boxes: { $elemMatch: { "box_id": "3000", "visible": true } } }, {"boxes.$":1});