Adding Field Separation for List Data

A Big List

MetalDetectr is effectively a list of data as specific as a user wishes to see. It will show only a list of releases a user has in his last.fm library to a list of everything on metal-archives.com. A big concern is presenting it properly, and one method is to delineate releases by whatever sort method a user wants to see. This can be by release date, by the band's name, by the release's name, or by the release's format (eg, EP, full-length, DVD).

The Algorithm

• Start with a table row showing the earliest or most recent, depending on sort order, of the selected sort column.
• Loop through the releases.
• If the current release's relevant field is greater/less than the preceding one, show another table row with the current release's field value.
• Show the release.

For example, the default sort is by US release date starting at the earliest date (and the current month so there's less noise). The list will display the current month and every album released during that month. When a release is next month, it will show next month and then all releases from that month. Continue on through the rest of the releases. If the user wants to see the list in descending order, it will show the last month first and work its way to the most current month.

The Code

First find the first value and display it in a full column span table row:

# views/releases/index,html.haml
- comparison_value = @releases.first.chain_methods(Release::FIELDS_WITH_METHODS[Release.default_sort(params[:s])])
= separator_row(comparison_value)

These two lines use the following methods:

# models/release.rb
# Sets the sort order to what's passed or us_date.
def self.default_sort(sort)
  sort || 'us_date'
end

# models/release.rb
# Takes an array of symbols and calls them on the release instance if it
# responds to them.
# Example: release.chain_methods([:us_date, :month]) => release.us_date.month
def chain_methods(methods)
  methods.inject(nil) do |memo, acc|
    target = memo ? memo : self
    target.respond_to?(acc) ? target.send(acc) : memo
  end
end

# helpers/releases_helper.rb
# Creates a row with a full colspan for the value.
def separator_row(value)
  value = Date::MONTHNAMES[value] if value.is_a?(Fixnum)
  content_tag(:tr, :class => cycle('even', 'odd')) do
    content_tag(:td, value, :class => 'separator_row', :colspan => 7)
  end
end

FIELDS_WITH_METHODS is a constant that contains a mapping of field names and methods to call on them to display properly:

# models/release.rb
FIELDS_WITH_METHODS = {
  'band' => [:band, :first, :downcase],
  'name' => [:name, :first, :downcase],
  'us_date' => [:us_date, :month],
  'euro_date' => [:euro_date, :month],
  'format' => [:format],
  nil => [:us_date, :month]
}

Then we loop through each release, updating the comparison value when we get to the next one:

# views/releases/index,html.haml
- @releases.each do |release|
  - current_value = release.chain_methods(Release::FIELDS_WITH_METHODS[Release.default_sort(params[:s])])
  - if Release.values_compared?(current_value, comparison_value, params[:d])
    - comparison_value = current_value
    = separator_row(comparison_value)
  - else
    - comparison_value = current_value
  = render release

Compare the two values based on the sort order:

# models/release.rb
# Sets the comparison operator to be greater than if the direction is nil or ascending,
# or less than if the direction is descending.
def self.comparison_operator(direction)
  (direction.nil? || direction == 'asc') ? :> : :<
end

# models/release.rb
# True if both value and comparison exist and
# if the direction is ascending:
#   true if value > comparison, false otherwise
# if the direction is descending:
#   true if value < comparison, false otherwise
def self.values_compared?(value, comparison, direction)
  value &&
  comparison &&
  value.send(
    Release.comparison_operator(direction),
    comparison
  )
end

We tried to abstract the comparisons and what's displayed so we can add new fields and only need to update the field-method mapping. There is always the possibility that a field is nil, since we don't always get all the possible data for every release, so chain_methods will call all the methods it can on a release instance until it finishes or returns nil. We could have chained a bunch of try()s together, but that didn't look right.

We also tried to get as much code out of the view as we could, and it can be improved, but it's okay for now.

Namespacing /lib Files and RSpec

I've been in an ongoing battle with RSpec to get it to properly load files in the /lib directory of a rails app. There's a class MetalArchivesFetcher wrapped in a MetalDetectr module as a namespace:

module MetalDetectr
  class MetalArchivesFetcher
    ...
  end
end

The spec file starts like this:

require 'spec_helper'
require 'metal_archives_fetcher'

describe MetalDetectr::MetalArchivesFetcher do
  ...
end

Without the require, I would receive the message, load_missing_constant': Expected /Users/danny/code/metaldetectr/lib/metal_archives_fetcher.rb to define MetalArchivesFetcher (LoadError) It felt a little off to need to require the file again because Rails already loads it in with config.autoload_paths += Dir["#{config.root}/lib/**/"] set in the config/application.rb file. I could put the require in spec_helper.rb, but it still felt strange.

I decided to remove the module namespace. That lets me remove the require line and all preceding MetalDetectr:: for every MetalArchivesFetcher call in the spec. Is this the right decision? It's definitely DRYer, but I do create tighter coupling. Jim Weirich's talk, "The Building Blocks of Modularity" (that I can't find online) does go over the trade-offs of writing code that is either more tightly or loosely coupled, and my takeaway from that is, since this file is already coupled to the application and models within it, why add an additional layer? It's more of a perceived loosening while only adding a bit more complexity. And that's usually not a good thing.

Perhaps I'll add it back in later, but for now, I'm not going to need it.