Saving/updating a model with nested attributes and single table inheritance

Question

I have a model, ModelRun, that accepts nested attributes for another model, ParameterValue. (ModelRun has_many :parameter_values.) However, ParameterValue also employs single-table inheritance to save two subclasses: NumericParameter and FileParameter. FileParameter uses CarrierWave to store a file.

The problem is that in ModelRunController when saving or updating a ModelRun, by default, @model_run.save or @model_run.update_attributes does not identify the type of ParameterValue attributes - it just tries to store them as ParameterValue. This works for NumericParameter values, but it raises an exception for FileParameters because the CarrierWave uploader doesn't get mounted to handle the file upload so ActiveRecord fails when trying to serialize the file to the database.

What's the cleanest way to handle this problem? The only solution that occurred to me was to manually populate the @model_run.parameter_values collection in the controller's create and update methods, since I can tell which type each ParameterValue should be and create the correct objects one by one. However, this seems like reimplementing a lot of Rails magic since I can't just use ModelRun.new(params[:model_run]) or @model_run.update_attributes anymore - seems like it throws away much of the advantage of using accepts_nested_attributes_for in the first place. Is there a better way, a Rails Way™?

Relevant parts of each model are copied below.

model_run.rb

class ModelRun < ActiveRecord::Base
  has_many :parameter_values, dependent: :destroy

  accepts_nested_attributes_for :parameter_values, allow_destroy: true

  attr_accessible :name, 
                  :description, 
                  :geometry_description, 
                  :run_date, 
                  :run_date_as_string, 
                  :parameter_values_attributes
end

parameter_value.rb

class ParameterValue < ActiveRecord::Base
  belongs_to :model_run

  attr_accessible :type,
                  :parameter_id, 
                  :description, 
                  :numeric_value,
                  :model_run_id,
                  :parameter_file
end

numeric_parameter.rb

class NumericParameter < ParameterValue
  attr_accessible :numeric_value
end

file_parameter.rb

class FileParameter < ParameterValue
  mount_uploader :parameter_file, ParameterFileUploader

  attr_accessible :parameter_file
end

parameter_file_uploader.rb

class ParameterFileUploader < CarrierWave::Uploader::Base
  storage :file

  def store_dir
    "#{Rails.root}/uploads/#{model.class.to_s.underscore}/#{model.id}"
  end

  def cache_dir
    "#{Rails.root}/tmp/uploads/cache/#{model.id}"
  end

end

Vlad Bogomolov · Accepted Answer · 2012-04-06 13:39:43Z

4

If i understand you well, you are trying to find convinient way of instantiating right subclass in STI hierarchy by passing :type?. If you don't need to change the type later, you can just add this hack to your ParameterValue class

class ParameterValue < ActiveRecord::Base
    class << self
      def new_with_cast(*attributes, &block)
        if (h = attributes.first).is_a?(Hash) && !h.nil? && (type = h[:type] || h['type']) && type.length > 0 && (klass = type.constantize) != self
          raise "wtF hax!!"  unless klass <= self
          return klass.new(*attributes, &block)
        end

        new_without_cast(*attributes, &block)
      end
      alias_method_chain :new, :cast

    end
  end

After this, passing right type will cause right ParameterValue instatntiating, including uploaders, validation etc.

edited Apr 6, 2012 at 13:39

answered Apr 6, 2012 at 6:29

Vlad Bogomolov

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1 Comment

Matt Winckler Over a year ago

Brilliant - way better than the hack I came up with, and taught me about metaclasses. Thanks!

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Saving/updating a model with nested attributes and single table inheritance

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