Ruby DataMapper一对多删除失败

从编辑的问题正文中复制答案，以便从“未回答”筛选器中删除此问题：

答案是在摧毁Beta后重新加载Alpha对象

class Alpha
  include DataMapper::Resource
  property :id,   Serial
  property :name, String
  has n, :betas
end

class Beta
  include DataMapper::Resource
  property :id,   Serial
  property :name, String
  belongs_to :alpha
end

# Create an Alpha with two Betas
@alpha = Alpha.new(:name => 'A')
@alpha.betas << Beta.new(:name => 'B')
@alpha.betas << Beta.new(:name => 'C')
@alpha.save

puts @alpha.betas.inspect
puts "Destroyed? #{@alpha.betas.first.destroy}"
puts @alpha.betas.inspect

puts @alpha.betas.inspect
puts "Destroyed? #{@alpha.betas.first.destroy}"
puts @alpha.betas.reload
puts @alpha.betas.inspect

---

~answer per

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puts @alpha.betas.inspect
puts "Destroyed? #{@alpha.betas.first.destroy}"
puts @alpha.betas.reload
puts @alpha.betas.inspect