Hi, I'm Nicholas Johnson!

software engineer / trainer / AI enthusiast

Creating new fields with $project

We can use project to add new fields to our documents based on expressions.

Say we had a list of people, like so:

  {firstName: "Chinstrap", surname: "McGee"}
  {firstName: "Bootle", surname: "Cheeserafter"}
  {firstName: "Mangstrang", surname: "Fringlehoffen"}

We can use project to compose a name field, like so:

  $project: {
    name: { $concat: ["$firstName", " ", "$surname"] },

This will give us results like this:

  { "name" : "Dave Smith" },
  { "name" : "Mike Moo" }

Exercise - String aggregation operators

We saw here how to use $concat to make a new attribute containing a concatenated string. Have a look at the other String aggregation operators here:


Attempt to capitalize all the names. This is useful because Mongo grouping and matching is case sensitive.

Conditional fields with $cond

We can set the value of a field using a boolean expression using cond.Thereareacoupleofwaystousecond. There are a couple of ways to use cond. You may wish to review the documentation here: http://docs.mongodb.org/manual/reference/operator/aggregation/cond/

Say we have a set of customers, and some of them have complaints. We might set a flag on all of the unhappy customers like so:

  $project: {
    unhappy: {
      $cond: { if: "$complaints", then: true, else: false },
    complaints: "$complaints",

Unhappy will either be true or false.

Exercise - Add a hasCat field

  • Use projection to add a hasCat field. This might form the basis for a future grouping or counting.

Exercise - Project the stocks

  • Modify your stocks. Project the profit as a profit field.
  • Add a isProfitable field to show if the profit is greater than 0.
  • Add a buyNow field to show if the profit is greater than 0.5

-# Answers -# db.people.aggregate( -# { -# project:{ -# hasCat: { -# cond: { if: ’cat', then: true, else: false } -# }, -# cat: 'cat’ -# } -# } -# )