How to detect an error in mapreduce - mongodb

Question

Let's have an "error_test.js" file that contains:
db = db.getMongo().getDB( "mydb" );
db.mycol.insert( { hello : "world" } );
print("it is shown");
db.runCommand(
{
mapReduce: "mycol",
map: function(){ print(not_exists); },
reduce: function(key, values){},
out: { replace: "myrescol" }
}
);
print("it is shown too (after error in mapreduce!)");
If I run the file (in Windows command line), I get:
mypath>mongo error_test.js
MongoDB shell version: 2.4.0
connecting to: test
it is shown
it is shown too (after error in mapreduce!)
mypath>echo %errorlevel%
0
mypath>
So we can deduce that:
the mapreduce error doesn't stop the execution.
the mapreduce error is not shown to the user.
the mapreduce error is not translated to the exit code (0 = success) (so a caller program can't detect the error).
The only way to know of the error is by looking for the following line at "mongod.log":
Wed Jun 12 10:02:37.393 [conn14] JavaScript execution failed: ReferenceError: not_exists is not defined near '(){ print(not_exists)'
Same happens if we use the "db.doEval(my_js)" method in Java and we put the content of "error_test.js" file into the "my_js" variable: The mapreduce error is not detected (no excepcion is launched, no null value is returned, "ok : 1.0" appears in the response).
So my question is: How can I detect an error in the mapreduce? (both in a js file and in the doEval() method)
Thank you

Answer 1

You need to capture the return document from db.runCommand() into a variable and then check its ok value in your script - you can then throw an error or print output, etc.
print("it is shown");
var res = db.runCommand(
{
mapReduce: "mycol",
map: function(){ print(not_exists); },
reduce: function(key, values){},
out: { replace: "myrescol" }
}
);
printjson(res);
if (res.ok == 0) {
print("Oopsy!");
throw("error! " + res.errmsg + " returned from mapreduce");
}
print("it is shown too (after error in mapreduce!)");