[{"items":[{"product":{"name":"prod1","price":{"pledge":1,"cost":19}}},{"product":{"name":"prod2","price":{"pledge":2,"cost":10}}}]}]

db.collection.update({},[{$set:{items:{$map:{input:"$items",in:{product:{$mergeObjects:["$$this.product",{"price":{$mergeObjects:["$$this.product.price",{deposit:"$$this.product.price.pledge"}]}}]}}}}}}])