1.0.0b3: regression with single-table inheritance and constraint added in a subclass

**Migrated issue, originally created by Gabor Gombas**

Hi,

The attached example works fine with 0.9.7, but 1.0.0b3 gives the following error: `sqlalchemy.exc.ArgumentError: Can't add unnamed column to column collection`

Gabor

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Attachments: [test.py](../wiki/imported_issue_attachments/3341/test.py)

