CODEWITHSUNDEEP
mongodbmongodb-queryaggregation-framework
Adam Rackis
Adam Rackis

Reputation: 83356

$dateToString on array items

If I have the following Mongo document

{
    "_id" : ObjectId("59e6cb45a902f0f3ea1e35cc"),
    "title" : "Book 1",
    "authors" : [ 
        {
            "name" : "Adam",
            "birthday" : ISODate("1982-03-22T22:41:19.198Z")
        }
    ]
}

How do I project the authors' birthday with $dateToString. Right now I have

db.getCollection('coll')
  .aggregate([
     {$match: {}}, 
     {$project: 
        {  
          title: 1, 
          authors: {
            name: 1, 
            birthday: { 
              $dateToString: { format: "%m", date: "$birthday" } 
            } 
          }
        } 
      }
])

but $birthday isn't finding the current author's birthday, and is returning null instead. And I've tried every other possibility I can think of. All either error out or return null.

Upvotes: 2

Views: 1192

Answers (1)

Neil Lunn
Neil Lunn

Reputation: 151122

Use $map for this sort of thing. It's usage is for reshaping arrays and some of the $project notation is a relatively new thing with only limited applications:

db.getCollection('coll').aggregate([
   { "$match": {}}, 
   { "$project": {  
     "title": 1, 
     "authors": {
       "$map": {
         "input": "$authors",
         "as": "item",
         "in": {
           "name": "$$item.name",
           "birthday": { 
             "$dateToString": { "format": "%m", "date": "$$item.birthday" }
           } 
         }
       }
     } 
   }}
 ])

That makes sure the operator is applied to the specific element of the array which is being processed, and returns the new array.

Upvotes: 3

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