TypeError：错误的参数类型String（预期的模块）

I have the following code:

我有以下代码:

class ProfileLookup < ActiveRecord::Base
  class << self
    ProfileLookup.select("DISTINCT category").map{|c| c.category}.each do |category|
      define_method("available_#{category.pluralize}".to_sym) do
        ProfileLookup.where(category: category).order(:value).all.collect{|g| g.value}
      end
    end
  end
end
class Pr