How to add new field with constant using $project?

Question

from doc:

The $addFields stage is equivalent to a $project stage that explicitly specifies all existing fields in the input documents and adds the new fields.

But if I write:

db.projects.aggregate(
        {
            $project: {             
                count: 1            
            }
        }  
)

This code doesn't add count to the result.

In contrast:

   db.projects.aggregate(
            {
                $addFields: {               
                    count: 1            
                }
            }  
    )

This adds count field to result.

How to add new field to result using $project? I think it is possible because it is equivalent.

AlexDenisov · Accepted Answer · 2017-08-07 13:14:58Z

4

You need to use $literal for this task:

db.projects.aggregate(
    {
        $project: {
            count: { $literal : 1 }
        }
    }
)

answered Aug 7, 2017 at 13:14

AlexDenisov

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