How to concat new string value with existing value in mongoengine while doing update?

Question

I want concat new string value with existing value.

suppose i have 3 document in my collection

{
   "field" : "some"
}
{
   "field" : "value"
}
{
   "field" : "test"
}

i want to write a update query which will concat "new" with existing value . My updated documents will look like

{
   "field" : "some new"
}
{
   "field" : "value new"
}
{
   "field" : "test new"
}