class Product < ActiveRecord::Base
  attr_accessible :name, :category_id
  validates :category, presence: true
  belongs_to :category
end

class Category < ActiveRecord::Base
  attr_accessible :name
  has_many :products
end

FactoryGirl.define do
  factory :category do
    name "MyString"
  end
end

FactoryGirl.define do
  factory :product do |f|
    name "MyString"
    category
  end
end