Mvc – Do Rails Join Models Get Controllers

That article is pretty much classic. Yes, you should stick to the "fat models, skinny controllers, stupid views" architecture as long as the models make sense as objects (and they almost always do).

In your case, I'm not really sure what the output "model" should look like, but let's say it's a gallery. So you can create a model like this:

class Gallery # no "< ActiveRecord" if don't work with database

  attr_accessor :zip_file_path, :xml, :images

  def initialize(params ={})
    @zip_file_path = params[:zip_file_path]
    @xml = nil
    @images = []
    @output = nil # model: html code, path to file...
  end

  def generate_model
    unzip_file! # populates @xml and @images
    transform_xml! # creates model into @output
    @output
  end

  def unzip_file!
    ...
  end

  def transform_xml!
    ...
  end

end

It's just a rough design, you might want to make it more function-oriented (@xml, @images = unzip_file(@zip_file_path) ) etc.

Controller will then act just as a mediator between model and views/user:

class GalleriesController < ApplicationController
  def new()
    # display upload form for zip_file
  end

 def create()
   # check input here, or leave it to model validations

  @output = Gallery.new(params).generate_model
 end

end

Martin Fowler's Patterns of Enterprise Application Architecture describes a number of ORM related paterns, including use of the Query Object, which is what I'd suggest.

Query objects let you follow the Single Responsibility principle, by separating the logic for each query into individually managed and maintained strategy objects. Either your controller can manage their use directly, or delegate that to a secondary controller or helper object.

Will you have a lot of them? Certainly. Can some be grouped into generic queries? Yes again.

Can you use dependency injection to create the objects from metadata? That's what most ORM tools do.