Introducing Hashdown

Solomon White — Tue, 04 Aug 2009 23:51:00 +0000

If your database is normalized, you will almost always end up with small tables (often referred to as reference data or lookup tables) which provide a set of possible values for a particular attribute. (e.g. Currency, Category, etc.) A common pattern that emerges in many applications is accessing these records by a symbolic name (as opposed to by id) for purposes of clarity when reading the code. In C (and friends), the database ids can be mapped to the positions of an enum datatype. I recently released the hashdown plugin that provides hash-like access for reference data records, and also adds some dropdown option list generation support, since this data is often used to populate select list in forms.

As an example of what hashdown does, suppose we have the following model:

class CardType < ActiveRecord::Base
end

with the following data:

+----+-------+------------------+
| id | code  | name             |
+====+=======+==================+
| 1  | visa  | Visa             |
| 2  | mc    | MasterCard       |
| 3  | disc  | Discover         |
| 4  | amex  | American Express |
+----+-------+------------------+

By adding the following line to the model:

class CardType < ActiveRecord::Base
  finder :code
end

You get the functionality of a hash-like square-bracket accessor for the model that will let you do something like:

@order.card_type = CardType[:visa]

The underlying implementation is similar to:

def CardType < ActiveRecord::Base
  def self.[](value)
    find_by_code(value)
  end
end

...except it adds a caching layer to boost performance by preventing repeated database access.

Adding the following directive:

def CardType < ActiveRecord::Base
  selectable
end

to the model gives you a class method called select_options that can be used to populate a select list like this:

<%= form.select :card_type_id, CardType.select_options %>

produces:

<input type="select" name="order[card_type_id]" id="order_card_type_id">
  <option value="1">Visa</option>
  <option value="2">MasterCard</option>
  <option value="3">Discover</option>
  <option value="4">American Express</option>
</input>

By default, this will use the id attribute as the submitted value of the option and call a display_name method (if it exists) for the displayed value of the option, falling back to the name method/attribute. Each of these can be overridden by passing a symbol attribute / method name, or a lambda that will be executed to generate the value. For (a contrived) example:

<%= form.select :card_type_id, CardType.select_options(:key => :code, :value => lambda{|card_type| card_type.name.reverse }) %>

produces:

<input type="select" name="order[card_type_id]" id="order_card_type_id">
  <option value="visa">asiV</option>
  <option value="mc">draCretsaM</option>
  <option value="disc">revocsiD</option>
  <option value="amex">sserpxE naciremA</option>
</input>

Again, the select_option results are cached for better performance.

This is a pretty small plugin that I’m using to DRY up some code in a current project I’m working on. Let me know if you have feature requests (or fork and patch it on GitHub!)

"Introducing Hashdown" by sksizer

Thu, 20 Aug 2009 04:24:42 +0000

Very nice – I’ve just been using static/class functions to achieve the same thing – cached values – but that’s obviously not so DRY. This is great!

"Introducing Hashdown" by sksizer

Thu, 20 Aug 2009 04:24:29 +0000

Very nice – I’ve just been using static/class functions to achieve the same thing – cached values – but that’s obviously not so DRY. This is great!

"Introducing Hashdown" by Solomon White

Sun, 09 Aug 2009 05:11:09 +0000

@Cássio—

Thanks—I’m glad you like it!

I’m using seed-fu to populate required data for my app, which I’ve liked pretty well so far.

"Introducing Hashdown" by Cássio Marques

Sat, 08 Aug 2009 23:54:24 +0000

Cool!

What method are you using to populate those tables? Are you using migrations for that? Or the populate plugin?

Thanks and good job!

OnRails.org: Introducing Hashdown

Introducing Hashdown

"Introducing Hashdown" by sksizer

"Introducing Hashdown" by sksizer

"Introducing Hashdown" by Solomon White

"Introducing Hashdown" by Cássio Marques