MongoDB Aggregation Framework: Understanding the $project Stage

What is the $project Stage?

The $project stage reshapes each document in the aggregation pipeline. It allows you to specify fields to keep, remove, or add new computed fields.

Unlike simple queries, $project works within aggregation pipelines to transform documents dynamically.

• Include fields by setting them to 1.
• Exclude fields by setting them to 0.
• Create new fields using expressions.
• Rename fields by assigning expressions to new field names.

Including and Excluding Fields

To include fields in the output, set their value to 1 in the $project document. To exclude, set the value to 0.

Note that you cannot mix inclusion and exclusion except for the _id field, which can be excluded explicitly.

• Include example: { name: 1, age: 1 }
• Exclude example: { password: 0, secret: 0 }
• Exclude _id explicitly with { _id: 0 }

Creating Computed Fields

You can add new fields or transform existing ones by using MongoDB expressions inside $project.

Expressions can perform arithmetic, string manipulation, conditional logic, and more.

• Add a new field: { fullName: { $concat: ["$firstName", " ", "$lastName"] } }
• Calculate age in months: { ageInMonths: { $multiply: ["$age", 12] } }
• Conditional field: { isAdult: { $gte: ["$age", 18] } }

Renaming Fields

To rename a field, create a new field with the desired name and assign the old field's value to it.

You can then exclude the original field if you don't want it in the output.

• Rename example: { newName: "$oldName", _id: 0 }
• Remember to exclude the original field if needed.

Example Usage of $project

Let's look at a practical example that demonstrates including fields, creating computed fields, and renaming.