下面这段代码，has_one 通过下面的代码，生成以下 3 个 API

build_association(attributes = {})
create_association(attributes = {})
create_association!(attributes = {})

代码位置：https://github.com/rails/rails/blob/v4.2.10/activerecord/lib/active_record/associations/builder/singular_association.rb#L15

def self.define_constructors(mixin, name)
  mixin.class_eval <<-CODE, __FILE__, __LINE__ + 1
    def build_#{name}(*args, &block)
      association(:#{name}).build(*args, &block)
    end

    def create_#{name}(*args, &block)
      association(:#{name}).create(*args, &block)
    end

    def create_#{name}!(*args, &block)
      association(:#{name}).create!(*args, &block)
    end
  CODE
end