R extend Method to dependend Namespace

Question

I've the following Problem:

I'm using the opencpu package to provide my R Package as a web application. In my package I created a RefClass lets call that

.A <- setRefClass(
  ".A",
  fields = c(
    id = "integer",
    text = "character"
  )
)

plus a constructor function:

A <- function(id,text ){return(.A(id,text))}

and on top I wrote a method "toJSON" for the class and also provided an S4 method like this:

.A$methods(
  toJSON = function(){
     return(sprintf('{\"id\": %s, \"text\": %s}',id,text))
})

  setMethod("toJSON", c(".A"),function(x,...){
    x$toJSON()
  })

So far everthing is fine. When I install the package and run opencpu, I can call the A method without a problem: (POST with parameters e.g.: {id: 123, text: "Hallo World"})

SERVERADRESS/ocpu/library/PACKAGENAME/R/A

But when I want the returned value to be directly converted into JSON I get the following error:

No method for S4 class:.A

A look at the opencpu site, tells that the procedure which is called in this case is:

library(jsonlite)
args <- fromJSON('{"id": 123, "text": "Hallo World"}')
output <- do.call(PKGNAME::A,args)
toJSON(output)

However this runs fine if I run it in a regular R session. But the error becomes reproducable if I change the last line from toJSON(output) to jsonlite::toJSON(output) Hence I think this might be the problem and I was wundering if I can add my "toJSON" S4 method with signature ".A" to the Namespace of "jsonlite" within my package?

Any Ideas?