Use $unwind operator with $project for to get the first element in an array. Let us create a collection with documents. Following is the query
>db.getFirstElementInArrayDemo.insertOne({"StudentName":"John","StudentSubject":["MongoDB","Python","MySQL"]}); { "acknowledged" : true, "insertedId" : ObjectId("5c9c41292d6669774125244e") } >db.getFirstElementInArrayDemo.insertOne({"StudentName":"Chris","StudentSubject":["Java","C"]}); { "acknowledged" : true, "insertedId" : ObjectId("5c9c413f2d6669774125244f") } >db.getFirstElementInArrayDemo.insertOne({"StudentName":"Robert","StudentSubject":["C++","Ruby"]}); { "acknowledged" : true, "insertedId" : ObjectId("5c9c41532d66697741252450") }
Following is the query to display all documents from a collection with the help of find() method
> db.getFirstElementInArrayDemo.find().pretty();
This will produce the following output
{ "_id" : ObjectId("5c9c41292d6669774125244e"), "StudentName" : "John", "StudentSubject" : [ "MongoDB", "Python", "MySQL" ] } { "_id" : ObjectId("5c9c413f2d6669774125244f"), "StudentName" : "Chris", "StudentSubject" : [ "Java", "C" ] } { "_id" : ObjectId("5c9c41532d66697741252450"), "StudentName" : "Robert", "StudentSubject" : [ "C++", "Ruby" ] }
Following is the query to get first element in an array and return using aggregate
> db.getFirstElementInArrayDemo.aggregate([ ... {$unwind:"$StudentSubject"}, ... {$group:{"_id":"$_id","FirstElement":{$first:"$StudentSubject"}}}, ... {$project:{"_id":0,"FirstElement":1}} ... ]);
This will produce the following output
{ "FirstElement" : "Java" } { "FirstElement" : "C++" } { "FirstElement" : "MongoDB" }
You can use $slice operator in find() to get the first element in the array. Following is the query
> db.getFirstElementInArrayDemo.find({},{"StudentSubject":{$slice:1}});
This will produce the following output
{ "_id" : ObjectId("5c9c41292d6669774125244e"), "StudentName" : "John", "StudentSubject" : [ "MongoDB" ] } { "_id" : ObjectId("5c9c413f2d6669774125244f"), "StudentName" : "Chris", "StudentSubject" : [ "Java" ] } { "_id" : ObjectId("5c9c41532d66697741252450"), "StudentName" : "Robert", "StudentSubject" : [ "C++" ] }