Update in forEach on mongodb shell

Question

I have got a collection aTable with 2 records:

 {
    \"title\" : \"record 1\",
    \"fields\" : [ 
        {
            \"_id\" : 1,
              


        

Answer 1

Starting Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update of a field based on its current value, and thus using a query instead of javascript:

// {
//   title: "record 1",
//   fields: [
//     { _id: 1, items: [1] },
//     { _id: 2, items: [2, 3, 4] },
//     { _id: 3, items: [5] }
//   ]
// }
db.collection.update(
  {},
  [{ $set: {
       fields: { $map: {
         input: "$fields",
         as: "x",
         in: {
           _id: "$$x._id",
           items: { $map: {
             input: "$$x.items",
             as: "y",
             in: { item: "$$y", key: 0 }
           }}
         }
       }}
  }}],
  { multi: true }
)
// {
//   title: "record 1",
//   fields: [
//     { _id: 1, items: [ { item: 1, key: 0 } ] },
//     { _id: 2, items: [ { item: 2, key: 0 }, { item: 3, key: 0 }, { item: 4, key: 0 } ] },
//     { _id: 3, items: [ { item: 5, key: 0 } ] }
//   ]
// }

• The first part {} is the match query, filtering which documents to update (in this case all documents).

• The second part [{ $set: { fields: { $map: { ... } } }] is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline):

  • $set is a new aggregation operator which in this case replaces the field's value.
  • Then (if we go passed the boilerplate) we're simply $mapping the two nested arrays.
  • More specifically, the second map transformation replaces [1, 2] by [{ item: 1, key: 0 }, { item: 2, key: 0 }].
• Don't forget { multi: true }, otherwise only the first matching document will be updated.