How to send RDS file to an R plumber API?

Question

I work with the R plumber API. I've created a very simple api that receives an RDS file (a data.frame). On this received rds, I perform a few basic operations and then return this modified data.frame.
With swagger, I can graphically test whether my API is working and it's not. I can't get the RDS file to work. I tried to follow the example here How to send rds file to plumber API using httr package ? but without success

Here below is my code :

library(plumber)
library(dplyr)
library(magrittr)
library(data.table)

#* Get dataframe and create feature
#* @parser multi
#* Add rds parser
#* @parser rds
#* @param df:file
#* @post /dataframe
function(req, res, df) {
  str(df)
  return(df[,1])
}

I get this error with swagger UI :

Answer 1

Insert a browser() in there to see why your code is failing:

function(req, res, df) {
  browser()
  str(df)
  return(df[,1])
}

For me, I generated started with saveRDS(head(mtcars), file="mt.rds"), then uploaded it. From here,

Browse[2]> df
$mt.rds
                     mpg cyl  disp  hp drat    wt  qsec vs am gear carb
Mazda RX4           21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag       21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
Datsun 710          22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive      21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout   18.7   8 360.0 175 3.15 3.440 17.02  0  0    3    2
Valiant             18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1

So you need to use df[[1]] (or a name if you know it):

function(req, res, df) {
  return(df[[1]][,1])
}

With that running, I'm able to do this:

library(httr)
library(magrittr)
res <- 
  POST(
    "http://127.0.0.1:9999/dataframe",
    body = list(
      # send the file with mime type `"application/rds"` so the RDS parser is used
      df = upload_file("mt.rds", "application/rds")
    )
  ) %>%
  content()
str(res)
# List of 6
#  $ : int 21
#  $ : int 21
#  $ : num 22.8
#  $ : num 21.4
#  $ : num 18.7
#  $ : num 18.1