add fields

Like $project.
Adds fields to a doc.
$project can remove fields.
$addFields can not.

example

project can take fields from original doc, selecting the fields we are looking to use.
addFields can make field changes/transformations ON the selected fields.
This is a style choice when performing many calcs.

Take nested data && convert to "flat" data in a single output object.
This does a few things

• explicitly names the fields to KEEP in the $project stage
• changes the values of those fields in the addField stage
  • addFields will reassign values of existing fields if the field name is already present through a previous pipeline: below, the gravity field gets passed through the first project stage, and gets reassigned in the addFields stage

db.solarSystem.aggregate([
  {
    $project: {
      _id: 0,
      name: 1,
      gravity: 1,
      mass: 1,
      radius: 1,
      sma: 1
    }
  },
  {
    $addFields: {
      gravity: "$gravity.value",
      mass: "$mass.value",
      radius: "$radius.value",
      sma: "$sma.value",
    }
  }
])