How to create string from {"jsonrpc" -> "2.0", "method" -> method, "params" -> {"filter" -> "{}"}}

Question

Consider the following function:

 f[json_] := URLFetch[
   "https://developers.betfair.com/api.betfair.com/exchange/betting/\
 json-rpc/v1",
   {"StatusCode", "Headers", "Content"},
   "Body" -> json,
   Method -> "POST",
   "Headers" -> {"X-Application" -> DEVKEY, 
   "X-Authentication" -> TOKEN}]

( TOKEN and DEVKEY have values but are private )

The function is called as follows:

 f["[
 {\"jsonrpc\": \"2.0\", 
 \"method\": \"SportsAPING/v1.0/listEventTypes\", 
 \"params\": {\"filter\":{}}}
 ]"]

I have a question about creating this json string.

Simplified I would like to be able to create this string from something like

 {"jsonrpc" -> "2.0", "method" -> method, "params" -> {"filter" -> "{}"}}

where method contains for example

 "SportsAPING/v1.0/listEventTypes\".

such that the URLFetch still works.

Is this possible?

(I tried several ToString's but I don't get it to work except if json contains exactly the value

 "[
 {\"jsonrpc\": \"2.0\", 
 \"method\": \"SportsAPING/v1.0/listEventTypes\", 
 \"params\": {\"filter\":{}}}
 ]" 

including escape characters.

A different URLFetch ( or 11.0 equivalent ) is acceptable.

Answer 1

Thanks to comments of @Kuba I was able to answer this question myself.

Note that the essence of the solution is the usage of

ExportString[string, "JSON", "Compact"->True]

Giving the function:

fetchData[json_] := ImportString[
  URLFetch[
    URLBETFAIR,
    {"StatusCode", "Headers", "Content"},
    "Body" -> json,
    Method -> "POST",
    "Headers" -> {"X-Application" -> DEVKEY, 
      "X-Authentication" -> TOKEN}][[3]],
  "JSON"]

The following solution works:

fetchData[ExportString[{
      {
       "method" -> "SportsAPING/v1.0/listCompetitions",
       "jsonrpc" -> "2.0",
       "params" -> {"filter" -> {"eventTypeIds" -> 
            getEventTypes[][[All, 1, 2]]}}
       }
      }, "JSON", "Compact" -> True]][[1, 2, 2]][[All, 1]][[All, 2]]

This is the solution I currently use:

getCompetitions[sel_] := fetchData[ExportString[{
       {
        "method" -> "SportsAPING/v1.0/listCompetitions",
        "jsonrpc" -> "2.0",
        "params" -> {"filter" -> {"eventTypeIds" -> sel}}
        }
       }, "JSON", "Compact" -> True]][[1, 2, 2]][[All, 1]][[All, 2]]