undefined Ruby中的类方法（在方法定义之前调用）

Question

我最后遇到了一个非常奇怪的错误：

当我运行此代码时：

class HomeFeature
    include Mongoid::Document
    include Mongoid::Timestamps
    include Mongoid::Bitfield

    belongs_to :home

    def self.amenities
        %w(lobby jacuzzi sauna cinema games roof_garden events_room tennis_court visits_space roaster green_areas cistern)
    end

    bitfield :amenities, *HomeFeature.amenities.map(&:to_sym)
end

它完美无缺，但是这个：

class HomeFeature
    include Mongoid::Document
    include Mongoid::Timestamps
    include Mongoid::Bitfield

    belongs_to :home

    bitfield :amenities, *HomeFeature.amenities.map(&:to_sym)

    def self.amenities
        %w(lobby jacuzzi sauna cinema games roof_garden events_room tennis_court visits_space roaster green_areas cistern)
    end
end

给我错误： ```NoMethodError：未定义的方法`设施＆＃39; for HomeFeature：Class````

有人可以解释为什么会这样吗？非常感谢

Answer 1

在第二个示例中，您尝试在定义类方法amenities之前调用它（它在行*HomeFeature.amenities.map(&:to_sym)上调用，但仅在稍后定义）。源代码的排序非常重要，因为代码是从上到下解析的。