slate blog, a rails cms | Custom configuration settings made easy

29 Oct

Custom configuration settings made easy

October 29th, 2007 — 7:00 pm — Chris

I’m currently working on extracting some of the business rules and options from slate into a separate configuration block. My first stab at this involved creating a custom ConfigurationBuilder class and using instance_eval to get some fancy DSL working. But then I thought of a much simpler solution: a custom Hash class that supported method-based access to keys. That is, hash.some_key would be the same as hash[:some_key].

Turns out, this really wasn’t too complicated. The slightly tricky part was handling nesting: I wanted my configuration options to be grouped, such as passwords.min_length. All I had to do for this to work was override the default method for my custom hash class.

The end result is as follows:

require 'active_support'
class Configuration
  cattr_accessor :settings

  # creates a new configuration object if
  # necessary
  def self.settings # !> redefine settings
    @@settings ||= ConfigurationHash.new
  end

  # returns the current configuration
  # by passing a block you can easily edit the
  # configuration values
  def self.config
    block_given? ? yield(self.settings) : self.settings
  end
end

# specialized hash for storing configuration settings
class ConfigurationHash < Hash
  # ensure that default entries always produce 
  # instances of the ConfigurationHash class
  def default(key=nil)
    include?(key) ? self[key] : self[key] = self.class.new
  end

  # provides member-based access to keys
  # i.e. params.id === params[:id]
  # note: all keys are converted to symbols
  def method_missing(name, *args)
    if name.to_s.ends_with? '=' 
      send :[]=, name.to_s.chomp('=').to_sym, *args
    else
      send(:[], name.to_sym)
    end    
  end
end

Here’s an example of how it works (say, in the environment.rb file):

Configuration.config do |config|
  config.passwords.min_length   = 8
  config.passwords.ttl          = 60
end

Configuration.config.passwords.min_length # => 8
Configuration.config.passwords.ttl # => 60

# some alternate ways to access the settings
Configuration.config[:passwords][:min_length] # => 8
Configuration.config.passwords[:ttl] # => 60

Just a note that the method chain can technically go on as long as you want:

Configuration.config.passwords.length.minimum = 8

This works because a new ConfigurationHash is created when you access a key that doesn’t exist.

There you have it – custom configuration settings made easy. One caveat: since this is all dynamic, if you spell a key wrong or provide an incorrect path, you won’t be getting the configuration setting you want. However, the trade-off is the ability to define new options instantly.

Nice. Did you consider @OpenStruct@ for the method-based access to the keys of a hash?

That would make sense, wouldn’t it? Honestly, though, that part (the method_missing) was easy. I’m sure it could be done either way. I’ll likely switch to a real class-based configuration with lots of @attr_accessor@ in the future simply for the benefit of easily documenting each of the properties. But this definitely works for now.

Congrats on the 100th comment ryan :)

Looks cool. One api suggestion: Configuration.config.passwords.min_length seems repetitive. Was there a purpose the .config. in there? Seems like it could be removed and then you have less key strokes. I’m lazy ;)

I thought about that, too. I almost mentioned (inquired?) about it the other day, but it’s typically safe to assume Chris is handling it in the slickest way possible. However, here’s my rough thought:

Using OpenStruct, could you do something like this?

config = OpenStruct.new(
  :pwd_min_length => Configuration.config.passwords.min_length,
  :pwd_ttl                 => Configuration.config.passwords.ttl
)

Then somehow make that available everywhere so you could refer to the Configuration as config.pwd_ttl or something.

For the grouping, you could probably come up with some sort of nested ostruct loop or something. But I haven’t really thought about that.

Just wondering what your thoughts were, and I if you’re actually using conditions like:

if params[:password].length < Configuration.config.passwords.min_length

But that might not even be how it works.

Ok, I’ll make everyone happy – add this to the Configuration class:

    def self.method_missing(name, *args, &block)
      self.config.send(name, *args, &block)
    end

Now, Configuration.users is the same as Configuration.config.users.

Just a note – I wasn’t being “snarky” (Dave said I was) :-)

Anyway, I have a new addition to ConfigurationHash:

    # retrieves the specified key and yields it
    # if a block is provided
    def [](key, &block)
      block_given? ? yield(super(key)) : super(key)
    end

Now, you can do things like this:

Configuration.cbscharf do |cbscharf|
  cbscharf.first_name = 'Chris'
  cbscharf.last_name  = 'Scharf'

  cbscharf.address do |address|
    address.city = 'Morgantown'
    address.state = 'WV'
  end
end

This is getting crazy!

When I first tried this it wasn’t working – the block was being ignored. Took my brain a couple of minutes to realise that the method_missing in ConfigurationHash needed updating to include an &block parameter:

def method_missing(name, *args, &block)
    if name.to_s.ends_with? '='
      send(:[]=, name.to_s.chomp('=').to_sym, *args)
    else
      send(:[], name.to_sym, &block)
    end
  end

Other than that, thanks for putting this together!

Very nice writeup. Thank you! I am looking into re-factoring much of my configuration and your article is definitely helpful.

Of course this is all dynamic, which is good and bad.
Some things you might want to be more permanent.

How about the ability load from and save to a .yml file?

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