---
title: Remember your MVCs
teaser:
tags: web,rails
author: Tammer Saleh
published_on: 2007-10-01
---

From novice to expert, I'm sure every Rails developer believes in <abbr
title="Model View Controller">MVC</abbr>.  But all to often I've seen
programmers cut right through the Model/View/Controller boundaries when
implementing new features - especially where precedent hasn't yet been set.

I can understand where the urge comes from, as there are times when the
boundaries are fuzzy.  Rails has the `validates_confirmation_of` for entering a
password and a password confirmation, while a good argument could be made that
this is logic that should never be in the model.  If the record was created by a
3rd party application, then it shouldn't have to specify the password twice in
the <abbr title="Extensible Markup Language">XML</abbr> just because the model
is _usually_ used by humans.  And the writers of the macro acknowledge this by
not having it kicking in if the field is nil.

A friend of mine recently proposed the following solution for creating a
[Combobox](https://en.wikipedia.org/wiki/Combobox) (a dropdown with a
customizable 'other' field).  He didn't like the complexity caused by using a
before filter on the controller, couldn't rely upon javascript in the form, and
wanted to keep his controllers virtually untouched.  Going on the skinny
controllers, fat models school of thought, he added the following in his model:

    attr_accessor :other_primary_source_of_funding
    attr_accessor :other_heard_through

    before_save :replace_with_others

    def replace_with_others
      unless other_primary_source_of_funding.blank?
        self.primary_source_of_funding = other_primary_source_of_funding
      end
      unless other_heard_through.blank?
        self.heard_through = other_heard_through
      end
    end

This code wins points in being concise and understandable, but is also a big 'ol
<abbr title="Model View Controller">MVC</abbr> violation.  The Right solution is
to grab the value in the controller:

    before_filter :convert_combobox_values, :only => [:create, :update]

    def convert_combobox_values
      if params[:object][:primary_source_of_funding] == "Other"
        params[:object][:primary_source_of_funding] = params[:object].delete(:other_primary_source_of_funding)
      end

      if params[:object][:heard_through] == "Other"
        params[:object][:heard_through] = params[:object].delete(:other_heard_through)
      end
    end

Here's the question you should be asking yourself when you think you're treading
the <abbr title="Model View Controller">MVC</abbr> line:  **Would the model need
this method if it wasn't being accessed through <abbr title="HyperText Markup
Language">HTML</abbr>?**  If you were using this model in a desktop application,
that question for the example above would be No.