Completing the Sponsor model and views

Have a look at [Rails Guides](http://guides.rubyonrails.org/association_basics.html#has-many-association-reference):

Indeed, `belongs_to :sponsor_type` sets up a few methods on Sponsor that allow you to create and access sponsor_type from instances of Sponsor. If you ever want to do the reverse, you need to add `has_many :sponsors` to the Sponsor_type model, which would set up the requisite methods on Sponsor_type instances. Then, if you have e.g. a 'corporate' instance of Sponsor_type, you can use `corporate.sponsors.where(donation_schedule: 'monthly')`.

So, it sounds like you already have need for `belongs_to` in the Sponsor model. Adding `has_many` to the Sponsor_type model cannot replace that, but gives you new functionality.