How to get property value direct from mongodb in JAVA

Question

Hi everyone I have a collection of documents like bellow. I want to directly get "rights" from roles array for params: _id, groups._id, roles._id using java mongo driver.

{
  "_id": 1000002,
  "groups": [
    {
      "_id": 1,
      "roles": [
        {
          "rights": 3,
          "_id": 1
        },
        {
          "rights": 7,
          "_id": 2
        },
        {
          "rights": 3,
          "_id": 3
        }
      ]
    }
  ],
  "timestamp": {
    "$date": {
      "$numberLong": "1675267318028"
    }
  },
  "users": [
    {
      "accessProviderId": 1,
      "rights": 1,
      "_id": 4
    },
    {
      "accessProviderId": 1,
      "rights": 3,
      "_id": 5
    }
  ]
}

I have AccessListItem class which represents this document and I have used Bson filters to get it from mongo, but after fetching i had to get information through java function.. I want to get int value directly from mongo base.

        Bson fileFilter = Filters.eq("_id", itemId);
        Bson groupFilter = Filters.elemMatch("groups", Document.parse("{_id:"+groupId+"}"));
        Bson roleFilter = Filters.elemMatch("groups.roles", Document.parse("{_id:"+role+"}"));

        Bson finalFilter = Filters.and(fileFilter, Filters.and(groupFilter,roleFilter));

        MongoCollection<AccessListItem> accessListItemMongoCollection =      MongoUtils.getAccessCollection(type);
        AccessListItem accessListItem =  accessListItemMongoCollection.find(finalFilter).first();

Answer 1

The short answer is you can't.

MongoDB is designed for returning documents, that is, objects containing key-value pairs. There is no mechanism for a MongoDB query to return just a value, i.e. it will never return just 3 or [3].

You could use aggregation with a $project stage at the end to give you a simplified object like:

{ rights: 3}

In javascript that might look like:

db.collection.aggregate([
  {$match: {
      _id: itemId,
      "groups._id": groupId,
      "groups.roles._id": role
  }},
  {$project: {
      _id: 0,
      group: {
        $first: {
          $filter: {
            input: "$groups",
            cond: {$eq: ["$$this._id",groupId]}
          }
        }
      }
  }},
  {$project: {
      "roles": {
        $first: {
          $filter: {
            input: "$group.roles",
            cond: { $eq: [ "$$this._id",role]}
          }
        }
      }
  }},
  {$project: {
      rights: "$roles.rights"
  }}
])

Example: Playground

I'm not familiar with spring boot, so I'm not sure what that would look like in Java.