在文档数组mongoose中获取精确的子文档

Question

I'm developing a graphql API using mongoose to interact with my mongo database and I'm struggling with something. I have documents with subdocuments arrays and I need to get all the documents containing a subdocument that have a specific field value. The problem is I want to get only this subdocument in the document. It's easier to explain with an example :

My documents are like that :

documents: [
{
  id: 1,
  items: [
    {
      value: 5,
    },
    {
      value: 10,
    },
  ]
},
{
  id: 2,
  items: [
    {
      value: 7,
    },
    {
      value: 10,
    },
  ]
}]

I want to get all the documents that have an item with value == 5 , but containing only this value like :

results : [
{
  id: 1,
  items: [
    {
      value: 5,
    }
  ]
}]

What I do for now is :

documents.find({
  items: {
    $elemMatch : {
      value: { $eq: 5 }
    }
  },
});

The thing is doing this I get the correct document, but it contains all the items, not only the ones that match the condition. I looked for a solution but I didn't find anything to do so.

Thanks for your help, I hope it is clear enough.

Answer 1

You would need to use the aggregation framework as follows:

db.test.aggregate(
{ $match: { "id": 1 }}, 
{ $unwind: '$items' }, 
{ $match: { "items.value": 5 }},
{ $project: { "_id": 0 }}
)

Return value:

{ "id" : 1, "items" : { "value" : 5 } }

Answer 2

You need to specify the fields you want to return from the query.

documents.find({
    items: {
      $elemMatch : {
        value: { $eq: 5 }
      }
    },
  }, {requiredFields: 1});

See example in this question.

Official docs: here - Search for "select"

If you need to filter out the results too, you an use the $filter