Getting only the first item for an array property in MongoDB?

Use $slice operator for this. Let us first create a collection with documents −

> db.gettingFirstItemInArrayDemo.insertOne(
   {
      "UserId": 101,
      "UserName":"Carol",
      "UserOtherDetails": [
         {"UserFriendName":"Sam"},
         {"UserFriendName":"Mike"},
         {"UserFriendName":"David"},
         {"UserFriendName":"Bob"}
      ]
   }
);
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cdfca52bf3115999ed51205")
}

Following is the query to display all documents from a collection with the help of find() method −

> db.gettingFirstItemInArrayDemo.find().pretty();

This will produce the following output −

{
   "_id" : ObjectId("5cdfca52bf3115999ed51205"),
   "UserId" : 101,
   "UserName" : "Carol",
   "UserOtherDetails" : [
      {
         "UserFriendName" : "Sam"
      },
      {
         "UserFriendName" : "Mike"
      },
      {
         "UserFriendName" : "David"
      },
      {
         "UserFriendName" : "Bob"
      }
   ]
}

Following is the query to get only the first item for an array property in MongoDB −

> db.gettingFirstItemInArrayDemo.find({"UserId":101}, {UserOtherDetails:{$slice: 1}});

This will produce the following output −

{ "_id" : ObjectId("5cdfca52bf3115999ed51205"), "UserId" : 101, "UserName" : "Carol", "UserOtherDetails" : [ { "UserFriendName" : "Sam" } ] }