我尝试从数据库中获取一些模型对象并将它们序列化为xml，但是当包含特定的（has_many）关联时，我得到以下错误。 JSON序列化（to_json）没有问题（包含属性）。还有一个对象的序列化（XML和JSON）。问题是什么？ （模型类不会覆盖任何XML序列化事物，我使用的是Rails 3）。
这里有命令：
Entity.all.to_xml :include => :properties
和转储：
NoMethodError: undefined method `macro' for nil:NilClass
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activesupport-3.0.1/lib/active_support/whiny_nil.rb:48:in `method_missing'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/serialization.rb:41:in `serializable_add_includes'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/serialization.rb:40:in `each'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/serialization.rb:40:in `serializable_add_includes'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/serializers/xml_serializer.rb:191:in `send'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/serializers/xml_serializer.rb:191:in `add_includes'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/serializers/xml_serializer.rb:186:in `add_extra_behavior'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activemodel-3.0.1/lib/active_model/serializers/xml.rb:103:in `serialize'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:134:in `call'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:134:in `_nested_structures'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:58:in `method_missing'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:31:in `tag!'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activemodel-3.0.1/lib/active_model/serializers/xml.rb:101:in `serialize'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/serializers/xml_serializer.rb:175:in `to_xml'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/associations/association_proxy.rb:218:in `send'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/associations/association_proxy.rb:218:in `method_missing'
... 5 levels...
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:134:in `call'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:134:in `_nested_structures'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:58:in `method_missing'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:31:in `tag!'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activemodel-3.0.1/lib/active_model/serializers/xml.rb:101:in `serialize'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activerecord-3.0.1/lib/active_record/serializers/xml_serializer.rb:175:in `to_xml'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activesupport-3.0.1/lib/active_support/xml_mini.rb:107:in `to_tag'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activesupport-3.0.1/lib/active_support/core_ext/array/conversions.rb:159:in `to_xml'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activesupport-3.0.1/lib/active_support/core_ext/array/conversions.rb:159:in `each'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activesupport-3.0.1/lib/active_support/core_ext/array/conversions.rb:159:in `to_xml'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:134:in `call'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:134:in `_nested_structures'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/builder-2.1.2/lib/builder/xmlbase.rb:58:in `method_missing'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activesupport-3.0.1/lib/active_support/core_ext/array/conversions.rb:158:in `__send__'
from /home/kai/.rvm/gems/ruby-1.8.7-p302@projectx/gems/activesupport-3.0.1/lib/active_support/core_ext/array/conversions.rb:158:in `to_xml'
HoBlend是对的......实体模型中的一个关联设置不正确，但没有properties一个。
在实体中，还存在与用户模型的关联：
belongs_to :created_by, :class_name => "User", :foreign_key => "created_by"
如果您习惯使用ActiveRecord关联，则会看到:foreign_key参数在此处无效（它必须位于关联的另一侧，它也是如此）。 Rails从不抱怨这个参数，我的测试都通过了。但它使XML序列化以某种方式失败（令人惊讶的不是JSON序列化）。
似乎ActiveRecord中存在某个错误。想想以下模型：
class User < ActiveRecord::Base
has_many :created_entities, :class_name => "Entity", :foreign_key => "created_by"
end
class Entity < ActiveRecord::Base
belongs_to :created_by, :class_name => "User", :foreign_key => "created_by"
end
实体迁移中存在created_by整数字段。当您将belongs_to关联命名为：foreign_key时，似乎XML序列化会中断。一旦我更改为belongs_to :creator, :class_name => "User", :foreign_key => "created_by"，序列化就会按预期工作。
这已经在Rails 3.0.3中解决了（我有3.0.1）。所以我该学到什么......首先更新软件！
答案 0 :(得分：1)
很可能模型关系设置不正确。 你能做到：
e = Entity.find.first e.properties
并让它返回关联的对象？