[{"ParentType":{"Food":[{"Name":"Burger","FoodId":"5e3abe145c1bfb31b4e335de","Price":0,"Quantity":1,"SubCategory":0}],"Inventory":[{"Name":"Small Popcorn","InventoryId":"5e3a64245c1bfb31b4e335b7","Price":0,"Quantity":1,"SubCategory":0},{"Name":"Small Popcorn2","InventoryId":"5e3a64245c1bfb31b4e335b7","Price":0,"Quantity":1,"SubCategory":0}]}},{"ParentType":{"Food":[{"Name":"Burger","FoodId":"5e3abe145c1bfb31b4e335de","Price":0,"Quantity":1,"SubCategory":0}],"Inventory":[{"Name":"Small Popcorn","InventoryId":"5e3a64245c1bfb31b4e335b7","Price":0,"Quantity":1,"SubCategory":0},{"Name":"Small Popcorn2","InventoryId":"5e3a64245c1bfb31b4e335b7","Price":0,"Quantity":1,"SubCategory":0}]}},]

db.collection.update({},[{$addFields:{"ParentType.Inventory":{"$map":{input:"$ParentType.Inventory",as:"i",in:{"$mergeObjects":["$$i",{UOM:null}]}}}}}],{multi:true})