Spring-MongoDB: debugging map\reduce on ubuntu.

while using Spring-data-MongoDB on ubuntu system, with default mongo installation you migh be experiencing troubles with error messages, as they might look similar to this:

InvalidDataAccessApiUsageException: Command execution failed:  Error [db assertion failure], Command = { "mapreduce" : ... }

The error message will include whole command, an will give you no clues about what actually went wrong. Try executing following command from terminal and repeat issue:

 tail -f /var/log/mongodb/mongodb.log 

trace information will get printed to log and you'll see it in terminal.  You should be looking for messages as following:

 JS Error: TypeError: myVar has no properties nofile_b:69

You can also add print to log in map\reduce functions with print("...") method invocation.

print('variable state is : ' + myVar) ;