Changing value in nested array from a scalar to an arry

[steevej (Steeve Juneau)]

Got some time to

@steevej

experiment with $map

See my answer to equivalent problem at Mongo query assistance update array element with field from object - #5 by steevej

Your problem is even simpler as you do not conditionally map. The following $set and $map should be good:

[ { "$set" :
    { "field_name.nested_array_of_obj" :
      { "$map" :
        { "input" : "$field_name.nested_array_of_obj" ,
          "as" : "item" ,
          "in" : { "field" : [ "$$item.field ] }
        }
      }
    }
  }
]