👐 Adding New Fields to Results

$addFields

We want to estimate the reading time for a book. But we don't have that field stored in our data. We can use $addFields for this. If the field exists, it'll get updated, and if it doesn't, it's added.

Atlas UI

[
    {$project: {
        title: 1,
        pages: 1,
        }
    },
    {$addFields: {readingTimeHours: {$divide: [{$multiply: ["$pages", 2]}, 60]}}},
]

MongoDB Shell

db.books.aggregate([
    {$project: {
        title: 1,
        pages: 1,
        }
    },
    {$addFields: {readingTimeHours: {$divide: [{$multiply: ["$pages", 2]}, 60]}}},
])

👐 Add a new field, notes, that contains the text PLACEHOLDER for all documents.

Answer

Atlas UI

[
    {$project: 
        {
            title: 1,
            pages: 1,
        }
    },
    {$addFields: {notes: "PLACEHOLDER"}}
]

MongoDB Shell

db.books.aggregate([
    {$project: 
        {
            title: 1,
            pages: 1,
        }
    },
    {$addFields: {notes: "PLACEHOLDER"}}
])

info

$set is an alias for $addFields that you'll find on many older posts and documentation.