Mongodb Using Projection And Sorting Complete Guide

Prerequisites:

• MongoDB installed on your local machine or access to a MongoDB Atlas instance.
• Basic understanding of MongoDB commands and data structure.
• Use the mongo shell or MongoDB Compass for executing queries.

Scenario:

Let's pretend you have a collection called students with the following documents:

{ "_id": 1, "name": "Alice", "age": 20, "gpa": 3.4, "major": "Computer Science" }
{ "_id": 2, "name": "Bob", "age": 22, "gpa": 3.2, "major": "Mathematics" }
{ "_id": 3, "name": "Charlie", "age": 21, "gpa": 3.8, "major": "Physics" }
{ "_id": 4, "name": "David", "age": 19, "gpa": 3.5, "major": "Computer Science" }
{ "_id": 5, "name": "Eve", "age": 23, "gpa": 3.7, "major": "Mathematics" }

Example 1: Basic Projection

In MongoDB, projection allows you to specify which fields to include or exclude from the result documents.

Objective: Retrieve the name and gpa of each student.

Without Projection:

db.students.find({})

This query will return all fields of every document in the students collection.

With Projection (Including only specific fields):

db.students.find({}, { name: 1, gpa: 1 })

This query will only return the name and gpa fields of each document. Note that _id is also included by default, unless you explicitly exclude it.

To also Exclude the _id Field:

db.students.find({}, { name: 1, gpa: 1, _id: 0 })

Expected Output:

{ "name": "Alice", "gpa": 3.4 }
{ "name": "Bob", "gpa": 3.2 }
{ "name": "Charlie", "gpa": 3.8 }
{ "name": "David", "gpa": 3.5 }
{ "name": "Eve", "gpa": 3.7 }

Example 2: Filtering with Projection

You might want to combine filtering (find() condition) with projection to retrieve specific details based on certain criteria.

Objective: Get the name and major of students older than 20.

Query:

db.students.find({ age: { $gt: 20 } }, { name: 1, major: 1, _id: 0 })

Expected Output:

{ "name": "Bob", "major": "Mathematics" }
{ "name": "Charlie", "major": "Physics" }
{ "name": "Eve", "major": "Mathematics" }

Example 3: Using _id in Projection

As mentioned earlier, the _id field is always included unless explicitly excluded. Here's how you can include or exclude the _id field.

Objective: Retrieve all student names without their IDs but include the major of those studying Computer Science.

Query (Exclude _id):

db.students.find({ major: "Computer Science" }, { name: 1, _id: 0 })

Expected Output:

{ "name": "Alice", "major": "Computer Science" }
{ "name": "David", "major": "Computer Science" }

Objective: Retrieve all student IDs and their gpa, excluding all other fields.

Query (Include _id and exclude others):

db.students.find({}, { _id: 1, gpa: 1 })

Expected Output:

{ "_id": 1, "gpa": 3.4 }
{ "_id": 2, "gpa": 3.2 }
{ "_id": 3, "gpa": 3.8 }
{ "_id": 4, "gpa": 3.5 }
{ "_id": 5, "gpa": 3.7 }

Example 4: Basic Sorting

You can sort the results returned by a query on one or more fields using sorting (sort() method).

Objective: Retrieve all student information, sorted by their gpa in ascending order.

Query:

db.students.find({}).sort({ gpa: 1 })

Here, 1 indicates ascending order. Alternatively, -1 signifies descending order.

Expected Output (Ascending Order):

{ "_id": 2, "name": "Bob", "age": 22, "gpa": 3.2, "major": "Mathematics" }
{ "_id": 4, "name": "David", "age": 19, "gpa": 3.5, "major": "Computer Science" }
{ "_id": 1, "name": "Alice", "age": 20, "gpa": 3.4, "major": "Computer Science" }
{ "_id": 5, "name": "Eve", "age": 23, "gpa": 3.7, "major": "Mathematics" }
{ "_id": 3, "name": "Charlie", "age": 21, "gpa": 3.8, "major": "Physics" }

Objective: Retrieve the name and gpa of all students, sorted by gpa in descending order.

Query:

db.students.find({}, { name: 1, gpa: 1, _id: 0 }).sort({ gpa: -1 })

Expected Output (Descending Order):

{ "name": "Charlie", "gpa": 3.8 }
{ "name": "Eve", "gpa": 3.7 }
{ "name": "David", "gpa": 3.5 }
{ "name": "Alice", "gpa": 3.4 }
{ "name": "Bob", "gpa": 3.2 }

Example 5: Combining Projection and Sorting

Sometimes, you need to use both projection and sorting together.

Objective: Find students who are majoring in Mathematics and sort them by gpa in descending order. Only display their name and gpa.

Query:

db.students.find({ major: "Mathematics" }, { name: 1, gpa: 1, _id: 0 }).sort({ gpa: -1 })

Expected Output:

{ "name": "Eve", "gpa": 3.7 }
{ "name": "Bob", "gpa": 3.2 }

Example 6: Sorting by Multiple Fields

You can sort documents based on multiple fields as well. For example, sorting primarily by one field and then by another in case of ties.

Objective: Find all students, sorted by their major in ascending order and then by their gpa in descending order. Only show their name, major, and gpa.

Query:

db.students.find({}, { name: 1, major: 1, gpa: 1, _id: 0 }).sort({ major: 1, gpa: -1 })

Expected Output:

{ "name": "Alice", "major": "Computer Science", "gpa": 3.4 }
{ "name": "David", "major": "Computer Science", "gpa": 3.5 }

{ "name": "Eve", "major": "Mathematics", "gpa": 3.7 }
{ "name": "Bob", "major": "Mathematics", "gpa": 3.2 }

{ "name": "Charlie", "major": "Physics", "gpa": 3.8 }

Example 7: Advanced Projection

You can also use projection to perform some advanced operations like displaying computed values using aggregation expressions.

However, the native find() method doesn't support computed fields directly. For this kind of operation, it’s common to use the Aggregation Framework. Here's a simple example using aggregation to project computed fields.

Objective: Find all students and project their name along with a computed field indicating whether their gpa is above the average.

First, Find the Average GPA:

avgGPA = db.students.aggregate([
    { $group: { _id: null, avgGPA: { $avg: "$gpa" } } }
]).toArray()[0].avgGPA

Next, Compare Each Student's GPA with the Average GPA: