$set

changes the values of fields. $project can show and hide fields, but it can't

set values of fields.

$project,

$set, and $addFields are all interchangable.

$set is used to create or change values of new

or existing fields. $project can be used to create or change the value of

fields, but it can also be used to specify which fields to show in the

documents in the aggregation pipeline.