How to Use $unwind Operator in MongoDB?

MongoDB is a NoSQL database widely used for its powerful aggregation framework which allows for complex data manipulation and analysis. One of the key operators in MongoDB’s aggregation pipeline is $unwind, which is used to deconstruct arrays within documents.

In this article, we will learn about the $unwind operator by understanding their examples in different situations and so on.

What is $unwind Operator?

• The $unwind operator in MongoDB’s aggregation framework is used to deconstruct an array field from the input documents to output a document for each element of the array.
• This is particularly useful when we want to perform operations on the individual elements of an array or when we need to level nested arrays within documents.

The syntax for the $unwind operator is as follows:

{ $unwind: "$arrayField" }

Explanation: Here the “arrayField” is the name of the array field to be deconstructed.

Examples of How to Use $unwind Operator in MongoDB

To understand How to Use $unwind Operator in MongoDB collection we need a collection and some documents on which we will perform various operations and queries. Here we will consider a collection called students which contains various information.

Output:

[
  {
    "_id": 1,
    "name": "Alice",
    "subjects": ["Math", "Science"]
  },
  {
    "_id": 2,
    "name": "Bob",
    "subjects": ["History", "Geography"]
  },
  {
    "_id": 3,
    "name": "Charlie",
    "subjects": ["Math", "English", "Physics"]
  },
  {
    "_id": 4,
    "name": "David",
    "subjects": ["Biology", "Chemistry"]
  },
  {
    "_id": 5,
    "name": "Eve",
    "subjects": ["Math", "Chemistry", "Physics"]
  }
]

Example 1: Counting the Number of Subjects for Each Student

• The below example uses the $unwind operator to deconstruct the subjects array and creating a separate document for each subject taken by each student.
• The $group stage then groups the documents by the student’s name ($name) and uses the $sum operator to count the number of subjects for each student.

db.students.aggregate([
  { $unwind: "$subjects" },
  { $group: { _id: "$name", totalSubjects: { $sum: 1 } } }
])

Output:

{ "_id": "Eve", "totalSubjects": 3 }
{ "_id": "Charlie", "totalSubjects": 3 }
{ "_id": "Bob", "totalSubjects": 2 }
{ "_id": "Alice", "totalSubjects": 2 }
{ "_id": "David", "totalSubjects": 2 }

Example 2: Finding the Most Common Subject Among Students

• This example also uses $unwind to deconstruct the subjects array.
• After that, the $group stage groups the documents by subject ($subjects) and uses the $sum operator to count the occurrences of each subject.
• The $sort stage then sorts the subjects based on their count in descending order (-1), and the $limit stage limits the output to the first document, which represents the most common subject

db.students.aggregate([
  { $unwind: "$subjects" },
  { $group: { _id: "$subjects", count: { $sum: 1 } } },
  { $sort: { count: -1 } },
  { $limit: 1 }
])

Output:

{ "_id": "Math", "count": 3 }

Example 3: Creating a List of Unique Subjects Across All Students

• This example uses $unwind to deconstruct the subjects array.
• The $group stage groups all the documents together (using _id: null) and uses the $addToSet operator to create an array of unique subjects across all students.

db.students.aggregate([
  { $unwind: "$subjects" },
  { $group: { _id: null, uniqueSubjects: { $addToSet: "$subjects" } } }
])

Output:

{ "_id": null, "uniqueSubjects": ["Math", "Science", "History", "Geography", "English", "Physics", "Biology", "Chemistry"] }

Conclusion

Overall, The $unwind operator is a powerful tool in MongoDB’s aggregation framework and it allowing for the efficient deconstruction of arrays within documents. Whether you’re working with single arrays, nested arrays, or performing advanced data analysis tasks, the $unwind operator provides a flexible solution. By mastering the usage of $unwind and combining it with other aggregation stages, you can perform complex data manipulation easily.