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addFields

Definition​

The addFields stage adds new fields to documents, the output from addFields are documents that contain all existing fields and newly added fields.

Read the official MongoDB documentation

Syntax​

addFields has the following form:

addFields varname=expression (,varname=expression)*

Behaviour​

addFields appends new fields to existing documents. You can include one or more addFields stages in an aggregation operation.

To add a field or fields to embedded documents (including documents in arrays) use the dot notation.

To add an element to an existing array field with addFields, use with concatArrays

Examples​

Using two addFields stages​

// Create a new collection in the private database called scores and add 2 documents
o1 = object as #Object
    student = "Maya"
    homework = [ 10, 5, 10 ]
    quiz = [ 10, 8 ]
    extraCredit = 0
end
o1.id = 1
save ${object:"scores"/o1}

o2 = object as #Object
    student = "Ryan"
    homework = [ 5, 6, 5 ]
    quiz = [ 8, 8 ]
    extraCredit = 8
end
o2.id = 2
save ${object:"scores"/o2}

// Use 2 addFields operations to include 3 new fields in the output documents
aggregate ${object:"scores"}
    addFields totalHomework = sum("$homework"), totalQuiz = sum("$quiz")
    addFields totalScore=add("$totalHomework","$totalQuiz","$extraCredit")
end

Adding fields to an embedded document​

Use dot notation to add new fields to embedded documents. This example create a collection called vehicles and adds a new field fuel_type to the embedded document specs

json = `
[
      { _id: 1, type: "car", specs: { doors: 4, wheels: 4 } },
      { _id: 2, type: "motorcycle", specs: { doors: 0, wheels: 2 } },
      { _id: 3, type: "jet ski" }
   ] 
`
objects = ${json:json}
save ${object:"vehicles"/objects}

aggregate ${object:"vehicles"}
    addFields "specs.fuel_type"="unleaded" 
end

Overwriting an existing field​

Specifying an existing field name in an addFields operation causes the original field to be replaced.

Using the prior example data in the vehicles collection.

aggregate ${object:"vehicles"}
    match _id=1
    addFields type="Motor Car" 
end

Add elements to an array​

Using the scores collection created earlier, you can use addFields with concatArrays to add an element to an existing array

aggregate ${object:"scores"}
    match _id=1
    addFields homework=concatArrays("$homework", [7]) 
end