RoR Achievement System - Polymorphic Association & Design Issues

Question

I\'m attempting to design an achievement system in Ruby on Rails and have run into a snag with my design/code.

Attempting to use polymorphic associations:

Answer 1

The way achievement systems generally work is that there are a large number of various achievements that can be triggered, and there's a set of triggers that can be used to test wether or not an achievement should be triggered.

Using a polymorphic association is probably a bad idea because loading in all the achievements to run through and test them all could end up being a complicated exercise. There's also the fact that you'll have to figure out how to express the success or failure conditions in some kind of table, but in a lot of cases you might end up with a definition that does not map so neatly. You might end up having sixty different tables to represent all the different kinds of triggers and that sounds like a nightmare to maintain.

An alternative approach would be to define your achievements in terms of name, value and so on, and have a constant table which acts as a key/value store.

Here's a sample migration:

create_table :achievements do |t|
  t.string :name
  t.integer :points
  t.text :proc
end

create_table :trigger_constants do |t|
  t.string :key
  t.integer :val
end

create_table :user_achievements do |t|
  t.integer :user_id
  t.integer :achievement_id
end

The achievements.proc column contains the Ruby code you evaluate to determine if the achievement should be triggered or not. Typically this gets loaded in, wrapped, and ends up as a utility method you can call:

class Achievement < ActiveRecord::Base
  def proc
    @proc ||= eval("Proc.new { |user| #{read_attribute(:proc)} }")
  rescue
    nil # You might want to raise here, rescue in ApplicationController
  end

  def triggered_for_user?(user)
    # Double-negation returns true/false only, not nil
    proc and !!proc.call(user)
  rescue
    nil # You might want to raise here, rescue in ApplicationController
  end
end

The TriggerConstant class defines various parameters you can tweak:

class TriggerConstant < ActiveRecord::Base
  def self.[](key)
    # Make a direct SQL call here to avoid the overhead of a model
    # that will be immediately discarded anyway. You can use
    # ActiveSupport::Memoizable.memoize to cache this if desired.
    connection.select_value(sanitize_sql(["SELECT val FROM `#{table_name}` WHERE key=?", key.to_s ]))
  end
end

Having the raw Ruby code in your DB means that it is easier to adjust the rules on the fly without having to redeploy the application, but this might make testing more difficult.

A sample proc might look like:

user.max_weight_lifted > TriggerConstant[:brickhouse_weight_required]

If you want to simplify your rules, you might create something that expands $brickhouse_weight_required into TriggerConstant[:brickhouse_weight_required] automatically. That would make it more readable by non-technical people.

To avoid putting the code in your DB, which some people may find to be in bad taste, you will have to define these procedures independently in some bulk procedure file, and pass in the various tuning parameters by some kind of definition. This approach would look like:

module TriggerConditions
  def max_weight_lifted(user, options)
    user.max_weight_lifted > options[:weight_required]
  end
end

Adjust the Achievement table so that it stores information on what options to pass in:

create_table :achievements do |t|
  t.string :name
  t.integer :points
  t.string :trigger_type
  t.text :trigger_options
end

In this case trigger_options is a mapping table that is stored serialized. An example might be:

{ :weight_required => :brickhouse_weight_required }

Combining this you get a somewhat simplified, less eval happy outcome:

class Achievement < ActiveRecord::Base
  serialize :trigger_options

  # Import the conditions which are defined in a separate module
  # to avoid cluttering up this file.
  include TriggerConditions

  def triggered_for_user?(user)
    # Convert the options into actual values by converting
    # the values into the equivalent values from `TriggerConstant`
    options = trigger_options.inject({ }) do |h, (k, v)|
      h[k] = TriggerConstant[v]
      h
    end

    # Return the result of the evaluation with these options
    !!send(trigger_type, user, options)
  rescue
    nil # You might want to raise here, rescue in ApplicationController
  end
end

You'll often have to strobe through a whole pile of Achievement records to see if they've been achieved unless you have a mapping table that can define, in loose terms, what kind of records the triggers test. A more robust implementation of this system would allow you to define specific classes to observe for each Achievement, but this basic approach should at least serve as a foundation.