[GH-302] by jonathaningram: Added failing test case for querying and joining on an associated inheri...

This issue is created automatically through a Github pull request on behalf of jonathaningram:

Url: https://github.com/doctrine/doctrine2/pull/302

Message:

...tance hierarchy

I have written this (partial) failing unit test. When running the test, you'll receive the Semantical Exception as follows:

```
[Semantical Error] line 0, col 81 near 'idd WHERE idd': Error: Class Doctrine\Tests\Models\Document\ImageDocument has no association named data
```

I expect that either:

1. This is expected behaviour. In which case, I'll update the test to expect the semantical exception.
2. This is a bug (I do hope it is!). In which case, any thoughts on a fix?

Either way, such a query would be great to have in the documentation (including the semantical error), so that when searching for the error, the Doctrine docs appear.

FYI, this has come up from my recent work on trying to find a solution for large inheritance hierarchies (and join limits) and solving the problem by using single table inheritance and associating a "data" class on the base class of the large hierarchy.

P.S. I hope that I do not have a flawed understanding of the ORM and that I'm trying to construct an impossible query/mapping. If so, apologies