function to insert a row in a table using json keys and data

postgres_json_insert.sql

CREATE OR REPLACE FUNCTION jsoninsert(relname text, reljson json)
  RETURNS record AS
$BODY$DECLARE
 ret RECORD;
 inputstring text;
BEGIN
  SELECT string_agg(quote_ident(key),',') INTO inputstring
    FROM json_object_keys(reljson) AS X (key);
  EXECUTE 'INSERT INTO '|| quote_ident(relname) 
    || '(' || inputstring || ') SELECT ' ||  inputstring 
    || ' FROM json_populate_record( NULL::' || quote_ident(relname) || ' , $1) RETURNING *'
    INTO ret USING reljson;
  RETURN ret;
END;
$BODY$
LANGUAGE plpgsql VOLATILE;