Generate new field in $project from the value of another - mongodb

Question

I have a collection of users with a field named level with a numeric value of 0 to 3. I am trying to return a generated field with the textual representation of the user's level. So far I have this, but it replaces the level field.
{ $project: {
'_id': 1,
'name': 1,
'email': 1,
'username': 1,
'password': 1,
'registered': 1,
'level': {
$switch: {
branches: [
{ case: 0, then: 'Pending' },
{ case: 1, then: 'Regular' },
{ case: 2, then: 'User manager' },
{ case: 3, then: 'Administrator' }
],
default: 'Unknown'
}
},
}
I would like to have a field named levelName in my aggregation output. How do I do this? I tried this:
'levelName': {
$switch: {
branches: [
{ case: { level: 0 }, then: 'Pending' },
{ case: { level: 1 }, then: 'Regular' },
{ case: { level: 2 }, then: 'User manager' },
{ case: { level: 3 }, then: 'Administrator' }
],
default: 'Unknown'
}
},
but to no avail.

Answer 1

In projection, you can easily change field name. For example, in your data, if you want to change your name of level field to levelName, you can simply write new name in field and assign value of old fieldName using $fieldName.
{
$project: {
levelName: "$level" //new name is levelName, and is assigned value of original field name level.
}
}
Now, in your case, you can achieve this by above way, However, your $switch statement is invalid or you are using $switch in wrong way. You need to specify which field to compare in your case expression i.e. { case: {$eq:["$level", 0]}, then: 'Pending' } instead of { case: 0, then: 'Pending' }.
db.indexName.aggregate([
{
$project:
{
'_id': 1,
'name': 1,
'email': 1,
'username': 1,
'password': 1,
'registered': 1,
levelName: {
$switch: {
branches: [
{ case: {$eq:["$level", 0]}, then: 'Pending' },
{ case: {$eq:["$level", 1]}, then: 'Regular' },
{ case: {$eq:["$level", 2]}, then: 'User manager' },
{ case: {$eq:["$level", 3]}, then: 'Administrator' }
],
default: 'Unknown'
}
}
}
}
])