How to just rename a field in an aggregation pipeline without removing other fields?-CodePudding

guys. I have an aggregation and a fields map, that I need to rename. Problem is, that the structure is not defined, so I don't really know which attributes will be in resulting output.

In the end, I need to rename fields without removing other ones. Example:

[
...
{
   field1: "value1", 
   field2: "value2",
   field3: "value2",
   field4: "value4",
}
...
]

For example, I want to rename output field field2 to myNewField. With $project I got:

[$project: {
  "myNewField": "$field2"       
}]

In this case only myNewField is displayed:

`
[
...
{ 
   myNewField: "value2"
}
...
]
`

Expected result:

[
...
{
   field1: "value1", 
   myNewField: "value2",
   field3: "value2",
   field4: "value4",
}
...
]

Thank you & have a nice day!

CodePudding user response：

You can add an stage just before the project using $set to add the new field and into the $project use the exclusion (0) with the field you don't want to output:

db.collection.aggregate([
  {
    "$set": {
      "myNewField": "$field2"
    }
  },
  {
    "$project": {
      "field2": 0
    }
  }
])

Example here