to_xml在通过Rails返回的对象上不起作用ActiveRecord habtm参考 [英] to_xml Doesn't Work on Objects Returned Through Rails ActiveRecord habtm Reference

问题描述

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

I have two rails active record classes, School and Instructor linked by a has_and_belongs_to_many relationship.

我需要查询我的教师学校特定学校的教师，并返回xml格式的响应.因此，在索引方法中，我有以下代码片段:

I need to query my instructors_controller for instructors for a particular school and return an xml format response. Therefore, in the index method I have this code fragment:

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

及更高版本:

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

但是它不起作用.看看这个有趣但令人困惑的序列:

But it doesn't work. Look at this intriguing but baffling sequence:

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" 

现在是重点:

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

这是怎么回事?

好吧，尽管我仍然想知道上述行为的解释，但从xml切换到json解决了这个问题(to_json在这里与to_xml并不一样).

Well, switching from xml to json solved the problem, (to_json does not exhibit the same strangeness as to_xml here), although I would still like to know the explanation for the above behavior.

而且，由于我是Rails的新手，是否有更好的方法来做我想在这里做的事情?

Also, as I am relatively new to Rails, is there a better way to do what I want to do here ?

推荐答案

to_xml()尝试为每个模型属性定义一个type属性.例如，具有年龄(INT)属性的用户模型将具有:<age type="integer">42</age>.此类型信息未使用JSON编码，这就是to_json()为您工作的原因.将此方法添加到您的讲师模型中:

to_xml() tries to define a type attribute for each model attribute. For example, a User model with an age (INT) attribute will have: <age type="integer">42</age>. This type information is not encoded in JSON, which is why to_json() works for you. Add this method to your Instructor model:

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，这将使您的XML缺少任何类型信息-如果某人拥有JAVA REST客户端，那就不好了.更好的策略是过滤self.attributes.slice(*keys).to_xml(to_xml_opts)，其中 keys 是XML中所需的模型属性数组.

This would however make your XML devoid of any type information - not good if someone has a JAVA REST client. A better strategy would be to filter self.attributes.slice(*keys).to_xml(to_xml_opts) where keys is an array of model attributes you want in the XML.

这篇关于to_xml在通过Rails返回的对象上不起作用ActiveRecord habtm参考的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！