has_many :options doesn't work

I am in the process of updating to 0.10.0.rc2 and I have an Option model when I try to serialize it with:

``` ruby
has_many :options
```

I get the error: 
`no implicit conversion of Symbol into Integer`.

If I rename the association via: 

``` ruby
has_many :foos, foreign_key: 'this_record_id', class_name: 'Option'
```

it works fine. 
BTW: I am upgrading from 0.9.3, hope that helps.
