to_xml Doesn't Work on Objects Returned Through Rails ActiveRecord habtm Reference

问题:

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

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 

and later:

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

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"  

Now for the punchline:

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 

What's going on here ?

Edit: 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.

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

回答1:

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 

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.

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