Preview an update to a Rails Model without saving anything to the database.

gistfile1.rb

# This class calls collection.build on each association that you specify in the @@variables,
# then calls assign_attributes on the attributes specific to this model.
#
# This class was created after I asked this question on stackoverflow.
# http://stackoverflow.com/questions/30109549/rails-preview-update-associations-without-saving-to-database

class PreviewEventUpdate
  attr_accessor :params, :nested_params, :event
  
  @@ids_keys        = [:audience_ids, :group_ids, :location_ids, :user_ids].freeze
  @@attributes_keys = [:occurrences_attributes, :ticket_types_attributes, :meta_urls_attributes].freeze

  def initialize(params, event = Event.new)
    @params, @event = params, event
    @nested_params  = {}
    separate_nested_ids
    separate_nested_attributes
  end

  def build
    @event.assign_attributes(@params)
    @nested_params.each do |key, attributes|
      assocation_name = key.to_s.gsub(/_id|_attributes/, '').to_sym
      @event.send(assocation_name).send(:build, attributes)
    end
    @event
  end

  private
    def separate_nested_attributes
      @@attributes_keys.each do |key|
        next unless @params[key]
        attributes = @params.delete(key)
        @nested_params[key] = remove_destroyed_associations(attributes.values)
      end
    end

    def separate_nested_ids
      @@ids_keys.each do |key|
        next unless @params[key]
        @nested_params[key] = attributes_from_ids(key, @params.delete(key))
      end
    end

    def attributes_from_ids(key, ids)
      ids.reject!(&:empty?)
      klass = key.to_s.split('_ids').first.classify.constantize
      klass.find(ids).map(&:attributes)
    end

    def remove_destroyed_associations(attributes)
      attributes.reject!{ |obj| obj[:_destroy] == "1" }
      attributes.each   { |obj| obj.delete(:_destroy) }
      attributes
    end
end