RCurl get response body when it errors out

Question

I'm just trying to get the response body when the post request fails using RCurl. When I'm running it through R, I only get the response error and it fails out.

Error: Unprocessable Entity

when I post the same request with the UI interface for testing, it gives

{
"reason": [
    "Can not create Data with Name: DataTest. Data Name should be unique."
],
"singleReason": "Can not create Data with Name: DataTest. Data Name should be unique."
}

Any help on how to get response bodies on 4XX errors is appreciated. TIA.

Post request

postdata.json <- '{"name":"DataTest","description":"Test Payload","algorithm":{"name":"DataTest","version":"0.1.0"}}'
post.result <- httpPOST(url=SERVER, postfields = postdata.json, verbose = T,
                        httpheader=c(Authorization=access.token, 'Content-Type'='application/json', Accept='application/json'))

RStudio output

* About to connect() to SERVER port 80 (#0)
*   Trying SERVER... * connected
* Connected to SERVER port 80 (#0)
> POST /api/test HTTP/1.1
Host: SERVER
Authorization: AUTHENTICATION
Content-Type: application/json
Accept: application/json
Content-Length: 171

< HTTP/1.1 422 Unprocessable Entity
< Content-Type: application/json;charset=UTF-8
< Date: Thu, 13 Nov 2014 16:31:42 GMT
< Server: Apache-Coyote/1.1
< Content-Length: 215
< Connection: keep-alive
< 
* Connection #0 to host SERVER left intact
 Show Traceback

 Rerun with Debug
 Error: Unprocessable Entity 

n.b. I'm okay with it failing and returning an Error. That's expected. I'm just trying to get the response body associated with the error code.

Answer 1

I was able to get the response body using RCurl by adding a basicTextGatherer() and a writefunction.

 reader <- basicTextGatherer()  
 post.result <- httpPOST(url=SERVER, postfields = postdata.json, verbose = T, 
                         writefunction = reader$update,
                         httpheader=c(Authorization=access.token, 
                                      'Content-Type'='application/json', 
                                      Accept='application/json')
                        )
 return(body=reader$value())

but Hadley's solution is simpler to work with and doesn't error out.

Answer 2

Here's one approach with httr:

library(httr)

postdata.json <- '{"name":"DataTest","description":"Test Payload","algorithm":{"name":"DataTest","version":"0.1.0"}}'

res <- POST(SERVER, 
  body = postdata.json, 
  add_headers(
    Authorization = access.token, 
    'Content-Type' = 'application/json', 
    Accept = 'application/json'
  )
)

http_status(res)
content(res)