Error is: Couldn't find Producttype without an ID

Rails
1

I am at a lost for what is going on here-------I have a producttypes and
categories table. prodtype_id is a foreign key in categories table that
references producttypes table. Below is how I am tyying to display it
and get the above error. Any suggestion

Product type:
<% @producttype.each do |producttype| %> <%= producttype.producttype_name %> <% end %>

Error is: Couldn’t find Producttype without an ID

Tables:

class CreateProducttypes < ActiveRecord::Migration
def self.up
create_table :producttypes do |t|
t.column :name, :string
t.column :created_at, :datetime
t.column :updated_on, :datetime

end

end

def self.down
drop_table :producttypes
end
End

class CreateCategories < ActiveRecord::Migration
def self.up
create_table :categories do |t|
t.column :name, :string
t.column :prodtype_id, :int
t.column :description, :string
t.column :created_at, :datetime
t.column :updated_on, :datetime
end
end

def self.down
drop_table :categories
end
end