Display foreign keys when explicitly instructed

given the following situation

```python
GENERAL, TYPE1 = range(2)


class SomeGeneralType(Base):
    __tablename__ = "some_general_table"
    id = Column(Unicode(255), primary_key=True)
    type = Column(Integer, nullable=False)
    __mapper_args__ = {
        "polymorphic_on": type,
        "polymorphic_identity": GENERAL,
    }

class Type1(SomeGeneralType):
    __tablename__ = "table1"
    id = Column(ForeignKey(SomeGeneralType.id), primary_key=True)
    __mapper_args__ = {
        "polymorphic_identity": TYPE1,
    }
```

since the ID is not auto-generated, user must to input it but it's also a foreign key and will be skipped before the patch.

And, some time there is just a foreign key without relationship defined... which is what I'm experiencing...



maybe also add a validator?