to_xml不适用于通过Rails返回的对象ActiveRecord habtm参考

Question

我有两个rails活动记录类,School和Instructor通过has_and_belongs_to_many关系链接.

我需要查询我的instructors_controller以获取特定学校的教师并返回xml格式的响应.因此,在索引方法中我有这个代码片段:

school = School.find(params[:school_id])
@instructors = school.instructors

然后:

respond_to do |format|
  format.html # index.html.erb
  format.xml  { render :xml => @instructors }
  format.json { render :json => @instructors }
end

但它不起作用.看看这个有趣但令人困惑的序列:

ruby-1.9.2-p180 :023 >   Instructor.all.first
=> #<Instructor id: 1, name: "Mrs. Smith", instructor_type_id: 1, created_at: "2011-07-24 18:19:40", updated_at: "2011-07-24 18:19:40"> 

ruby-1.9.2-p180 :026 > Instructor.all.first.class
 => Instructor(id: integer, name: string, instructor_type_id: integer, created_at: datetime, updated_at: datetime) 

ruby-1.9.2-p180 :024 > School.first.instructors.first
 => #<Instructor id: 1, name: "Mrs. Smith", instructor_type_id: 1, created_at: "2011-07-24 18:19:40", updated_at: "2011-07-24 18:19:40"> 

ruby-1.9.2-p180 :025 > School.first.instructors.first.class
 => Instructor(id: integer, name: string, instructor_type_id: integer, created_at: datetime, updated_at: datetime) 

ruby-1.9.2-p180 :021 > Instructor.all.first.to_xml
 => "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<instructor>\n  <created-at type=\"datetime\">2011-07-24T18:19:40Z</created-at>\n  <id type=\"integer\">1</id>\n  <instructor-type-id type=\"integer\">1</instructor-type-id>\n  <name>Mrs. Smith</name>\n  <updated-at type=\"datetime\">2011-07-24T18:19:40Z</updated-at>\n</instructor>\n" 

现在为了妙语:

ruby-1.9.2-p180 :019 > School.first.instructors.first.to_xml

NoMethodError: undefined method `type' for nil:NilClass
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activesupport-3.0.9/lib/active_support/whiny_nil.rb:48:in `method_missing'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activerecord-3.0.9/lib/active_record/serializers/xml_serializer.rb:230:in `compute_type'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activemodel-3.0.9/lib/active_model/serializers/xml.rb:22:in `initialize'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activemodel-3.0.9/lib/active_model/serializers/xml.rb:75:in `new'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activemodel-3.0.9/lib/active_model/serializers/xml.rb:75:in `block in serializable_attributes'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activemodel-3.0.9/lib/active_model/serializers/xml.rb:74:in `each'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activemodel-3.0.9/lib/active_model/serializers/xml.rb:74:in `map'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activemodel-3.0.9/lib/active_model/serializers/xml.rb:74:in `serializable_attributes'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activemodel-3.0.9/lib/active_model/serializers/xml.rb:116:in `add_attributes_and_methods'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activemodel-3.0.9/lib/active_model/serializers/xml.rb:103:in `block in serialize'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/builder-2.1.2/lib/builder/xmlbase.rb:134:in `call'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/builder-2.1.2/lib/builder/xmlbase.rb:134:in `_nested_structures'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/builder-2.1.2/lib/builder/xmlbase.rb:58:in `method_missing'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/builder-2.1.2/lib/builder/xmlbase.rb:31:in `tag!'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activemodel-3.0.9/lib/active_model/serializers/xml.rb:102:in `serialize'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/activerecord-3.0.9/lib/active_record/serializers/xml_serializer.rb:175:in `to_xml'
    from (irb):19
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/railties-3.0.9/lib/rails/commands/console.rb:44:in `start'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/railties-3.0.9/lib/rails/commands/console.rb:8:in `start'
    from /usr/local/rvm/gems/ruby-1.9.2-p180@blueprint/gems/railties-3.0.9/lib/rails/commands.rb:23:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'ruby-1.9.2-p180 :020 > Instructor.all.first.to_xml

这里发生了什么 ？

编辑:好吧,从xml切换到json解决了问题,(to_json在这里没有表现出与to_xml相同的陌生感),尽管我仍然想知道上述行为的解释.

另外,由于我对Rails相对较新,有没有更好的方法来做我想做的事情？

Answer 1

to_xml()尝试为每个模型属性定义一个type属性.例如,具有age(INT)属性的User模型将具有:<age type="integer">42</age>.此类型信息不是以JSON编码的,这就是为什么to_json()适合您.将此方法添加到您的Instructor模型:

def to_xml(options = {})
  to_xml_opts = {:skip_types => true} # no type information, not such a great idea!
  to_xml_opts.merge!(options.slice(:builder, :skip_instruct))
  # a builder instance is provided when to_xml is called on a collection of instructors,
  # in which case you would not want to have <?xml ...?> added to each item
  to_xml_opts[:root] ||= "instructor"
  self.attributes.to_xml(to_xml_opts)
end

但是这会使您的XML缺少任何类型信息 - 如果有人拥有JAVA REST客户端则不好.更好的策略是过滤self.attributes.slice(*keys).to_xml(to_xml_opts)哪里的键是XML中所需的模型属性数组.