$project stage for shaping output in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the $project stage in a MongoDB aggregation pipeline?

The $project stage is used to shape the output documents by including, excluding, or adding new fields. It controls which fields appear in the final result.

beginner

How do you exclude a field named password using $project?

Set the field to 0 in $project: <br>{ password: 0 } excludes the password field from the output.

intermediate

How can you rename a field firstName to name using $project?

Use the original field as the value for the new field: <br>{ name: "$firstName" } creates a new field name with the value from firstName.

intermediate

Can you add a new field with a computed value in $project? Give an example.

Yes. For example, to add a field fullName by combining firstName and lastName:<br>{ fullName: { $concat: ["$firstName", " ", "$lastName"] } }

advanced

What happens if you include one field with 1 and exclude another with 0 in the same $project stage?

MongoDB does not allow mixing inclusion (1) and exclusion (0) except for the _id field. You must choose either to include fields or exclude fields, but not both.

What does { age: 1 } mean inside a $project stage?

A Create a new field named <code>age</code>

B Exclude the <code>age</code> field from the output

C Rename the <code>age</code> field

D Include the <code>age</code> field in the output

Setting a field to 1 in $project means to include that field in the output documents.

How do you exclude the password field in $project?

A { password: false }

B { password: 1 }

C { password: 0 }

D { password: null }

Setting a field to 0 in $project excludes it from the output.

Which of the following is a valid way to add a new field fullName by combining firstName and lastName?

A { fullName: { $concat: ["$firstName", " ", "$lastName"] } }

B { fullName: "$firstName" & "$lastName" }

C { fullName: "$firstName" + "$lastName" }

D { fullName: "$firstName $lastName" }

The $concat operator combines strings in $project to create a new field.

What is the default behavior of _id in $project if not specified?

A It is excluded by default

B It is included by default

C It causes an error

D It is renamed to <code>id</code>

The _id field is included by default unless explicitly excluded.

Is this $project valid? { name: 1, age: 0 }

A No, mixing inclusion and exclusion is not allowed

B Yes, it includes <code>name</code> and excludes <code>age</code>

C Yes, but only if <code>_id</code> is excluded

D No, fields must be set to true or false

MongoDB does not allow mixing inclusion and exclusion in the same $project stage except for _id.

Explain how the $project stage shapes the output in a MongoDB aggregation pipeline.

Think about how you can choose which data to see and how it looks.

Describe the rules about mixing field inclusion and exclusion in the $project stage.

Remember the special case for the _id field.