vibe.d json deserializeJson!Foo and ddbc.update!Foo

[Chris Bare]

I'm working on a project with vibe.d and ddbc to access my 
database.
Both have powerful functions that operate on Struct types to 
avoid a lot of boilerplate code.
I have a web site that sends a json message to update a record.
deserializeJson creates a new Foo record containing the json data.
However, there are some fields in the record which are not in the 
json, and these get default values.
I have another Foo that has the original values.

What would be the best way to merge the updated values into the 
original?
Obviously I can copy from one Foo to the other, but that's pretty 
inelegant.
If I could pass an existing Foo to deserializeJson and have only 
some fields filled in, that would be perfect.
I tried this:

deserializeJson!Foo(foo, req.json);

but it completely overwrites all the fields in foo.

Any suggestions? Is there a trick I'm just not seeing?

--
Chris