Right way to do the lookup

Working with Data
1

I have a driver collection called bom which has

{
     _id
     name
     parts: [{ 
               partId : ObjectId(),
               extras: {
                        quantity: 20,
                        notes: “”,
                        uom: “unit”
                        } }
              ]
}

the lookup collection is parts where i have part details

when i do lookup the field extras from part array of bom collection is lost

how do i just replace partId in the array from lookup collection abd maitain all others efficiantly

output i get

  {
     _id
     name
     parts: [{ 
               part object from lookup,
             }
            ]
   }

output i want

 {
     _id
     name
     parts: [{ 
               part object from lookup,
               extras: {
                        quantity: 20,
                        notes: “”,
                        uom: “unit”
                        } }
              ]
}