如何在 R 中使用 Plumber 使用一组 JSON 对象

Question

I have been experimenting with Plumber in R recently, and am having success when I pass the following data using a POST request;

{"Gender": "F", "State": "AZ"}

This allows me to write a function like the following to return the data.

#* @post /score
    score <- function(Gender, State){
      data <- list(
        Gender = as.factor(Gender)
        , State = as.factor(State))

      return(data)
      }

However, when I try to POST an array of JSON objects, I can't seem to access the data through the function

[{"Gender":"F","State":"AZ"},{"Gender":"F","State":"NY"},{"Gender":"M","State":"DC"}]

I get the following error

{
    "error": [
        "500 - Internal server error"
    ],
    "message": [
        "Error in is.factor(x): argument \"Gender\" is missing, with no default\n"
    ]
}

Does anyone have an idea of how Plumber parses JSON? I'm not sure how to access and assign the fields to vectors to score the data.

Thanks in advance

Answer 1

I see two possible solutions here. The first would be a command line based approach which I assume you were attempting. I tested this on a Windows OS and used column based data.frame encoding which I prefer due to shorter JSON string lengths. Make sure to escape quotation marks correctly to avoid 'argument "..." is missing, with no default' errors:

curl -H "Content-Type: application/json" --data "{\"Gender\":[\"F\",\"F\",\"M\"],\"State\":[\"AZ\",\"NY\",\"DC\"]}" http://localhost:8000/score
# [["F","F","M"],["AZ","NY","DC"]]

The second approach is R native and has the advantage of having everything in one place:

library(jsonlite)
library(httr)

## sample data
lst = list(
  Gender = c("F", "F", "M")
  , State = c("AZ", "NY", "DC")
)

## jsonify
jsn = lapply(
  lst
  , toJSON
)

## query
request = POST(
  url = "http://localhost:8000/score?"
  , query = jsn # values must be length 1
)

response = content(
  request
  , as = "text"
  , encoding = "UTF-8"
)

fromJSON(
  response
)
#      [,1]                    
# [1,] "[\"F\",\"F\",\"M\"]"   
# [2,] "[\"AZ\",\"NY\",\"DC\"]"

Be aware that httr::POST() expects a list of length-1 values as query input, so the array data should be jsonified beforehand. If you want to avoid the additional package imports altogether, some system() , sprintf() , etc. magic should do the trick.

Finally, here is my plumber endpoint (living in R/plumber.R and condensed a little bit):

#* @post /score
score = function(Gender, State){
  lapply(
    list(Gender, State)
    , as.factor
  )
}

and code to fire up the API:

pr = plumber::plumb("R/plumber.R")
pr$run(port = 8000)