I think your trouble here is with the polymorphic association. (Why do you need it - is there another model that has_one or has_many :internships :as => :employer?)
Try either removing :polymorphic => true if you don't need it, or adding :as => :employer to the Employer's has_many :internships line.
http://wiki.rubyonrails.org/rails/pages/UnderstandingPolymorphicAssociations