Use a field's value and add it to a new field in an array - Mongodb

Question

I am attempting to turn this

{
   "fooList" : [
      {
         "bar" : {
            "baz" : 100
         }
      },
      {
         "bar" : {
            "baz" : 200
         }
      }
   ]
},
{
   "fooList" : [
      {
         "bar" : {
            "baz" : 300
         }
      },
      {
         "bar" : {
            "baz" : 400
         }
      }
   ]
}

into this:

{
   "fooList" : [
      {
         "baz" : 100,
         "bar" : {
            "baz" : 100
         }
      },
      {
         "baz" : 200,
         "bar" : {
            "baz" : 200
         }
      }
   ]
},
{
   "fooList" : [
      {
         "baz" : 300,
         "bar" : {
            "baz" : 300
         }
      },
      {
         "baz" : 400,
         "bar" : {
            "baz" : 400
         }
      }
   ]
}

As you can see, I really just copy baz and its value out of bar, but my issue is that is occurs within an array.

db.getCollection(<collection_name>).updateMany(
    {}, 
    { $set: { 'fooList.$[element].baz' : '$fooList.$[element].bar.baz' } },
    { upsert: true ,
      arrayFilters: [{'element.bar' : { $exists : true }}]
    }
)

But this will only set the string $fooList.$[element].bar.baz into baz, seen here through the results

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "fooList": [
      {
        "bar": {
          "baz": 100
        },
        "baz": "$fooList.$[element].bar.baz"
      }
    ]
  }
]

Can anyone tell me what I might be doing wrong, or if this is even possible? Thanks

Answer 1

Here is a variation that demonstrates the v4.4 "merge onto self" capability that turns aggregate into an update. This is a useful approach when you want to process all the docs because it eliminates the "no filter" ({}) and {multi:true} bits required for update.

db.foo.aggregate([
    // The $map/$mergeObjects is the SAME as the answer above, just with a little
    // more compact format.  It is import to $project here, NOT $addFields, because
    // we seek to limit the pipeline to just _id (comes along automatically) and 
    // the fooList:
    {$project: {
        fooList: {$map: {
            input: "$fooList",
                in: {$mergeObjects: [
                        "$$this",
                        {$cond: [
                            {$ne: ["$$this.bar.baz", undefined]},
                            {baz: "$$this.bar.baz"},
                            {}  // else empty object                                          
                        ]}
                     ]}
        }}
    }}

    //  ...and now, using _id as the key (fast!), merge fooList back into doc:
    ,{$merge: {
        into: "foo",
        on: [ "_id" ],
        whenMatched: "merge",
        whenNotMatched: "fail"
    }}
]);