$addFields for computed fields in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $addFields stage do in a MongoDB aggregation pipeline?

It adds new fields or modifies existing fields in the documents by computing values, without removing any existing fields.

beginner

How can you use $addFields to create a new field that is the sum of two existing fields, price and tax?

Use $addFields with an expression like: { totalCost: { $add: ["$price", "$tax"] } } to add a new field totalCost.

beginner

Can $addFields remove fields from documents?

No, $addFields only adds or updates fields. To remove fields, use $project or $unset.

intermediate

What happens if you use $addFields to add a field that already exists in the document?

The existing field is overwritten with the new computed value.

intermediate

Give an example of using $addFields to add a field isAdult that is true if age is 18 or more, false otherwise.

Use: { $addFields: { isAdult: { $gte: ["$age", 18] } } }. This adds isAdult as true or false based on age.

What is the main purpose of the $addFields stage in MongoDB?

A Sort documents

B Add or update fields in documents

C Remove fields from documents

D Group documents

$addFields adds new fields or updates existing ones without removing any fields.

Which operator can you use inside $addFields to sum two fields price and tax?

A $multiply

B $sum

C $add

D $concat

$add is used to add numeric values inside $addFields.

If you want to remove a field from documents, which stage should you use instead of $addFields?

A $unset

B $addFields

C $group

D $match

$unset removes fields from documents.

What happens if $addFields adds a field that already exists?

A The existing field is overwritten

B The field is ignored

C The field is duplicated

D It causes an error

The existing field is replaced with the new computed value.

How would you add a boolean field isAdult that is true if age is 18 or more?

A { isAdult: { $lt: ["$age", 18] } }

B { isAdult: { $ne: ["$age", 18] } }

C { isAdult: { $eq: ["$age", 18] } }

D { isAdult: { $gte: ["$age", 18] } }

$gte checks if age is greater than or equal to 18.

Explain how $addFields can be used to compute new fields in MongoDB documents.

Think about how you can add new information to each document without losing old data.

Describe a scenario where $addFields is useful in a data aggregation pipeline.

Consider when you want to enrich your data with new calculated details.