Github user StephanEwen commented on the pull request:
https://github.com/apache/incubator-flink/pull/141#issuecomment-57940003
The error message *"Type GenericType<test.PoJoTestProgram.MyType> is not a
composite type. Key expressions are not supported."* is not very helpful, IMHO.
How about writing something like `Key expressions are only supported on
POJO types. A type is considered a POJO if all its fields are public, or have
bother getters and setters defined`. That would immediately tell the user where
to look for the error, or how to alter his types.
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