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:

class Achievement < ActiveRecord::Base
  belongs_to :achievable, :polymorphic => true
end

class WeightAchievement < ActiveRecord::Base
  has_one :achievement, :as => :achievable
end

Migrations:

class CreateAchievements < ActiveRecord::Migration
... #code
    create_table :achievements do |t|
      t.string :name
      t.text :description
      t.references :achievable, :polymorphic => true

      t.timestamps
    end

     create_table :weight_achievements do |t|
      t.integer :weight_required
      t.references :exercises, :null => false

      t.timestamps
    end
 ... #code
end

Then, when I try this following throw-away unit test, it fails because it says that the achievement is null.

test "parent achievement exists" do
   weightAchievement = WeightAchievement.find(1)
   achievement = weightAchievement.achievement 

    assert_not_nil achievement
    assert_equal 500, weightAchievement.weight_required
    assert_equal achievement.name, "Brick House Baby!"
    assert_equal achievement.description, "Squat 500 lbs"
  end

And my fixtures:
achievements.yml…

BrickHouse:
 id: 1
 name: Brick House
 description: Squat 500 lbs
 achievable: BrickHouseCriteria (WeightAchievement)

weight_achievements.ym…

 BrickHouseCriteria:
     id: 1
     weight_required: 500
     exercises_id: 1

Even though, I can’t get this to run, maybe in the grand scheme of things, it’s a bad design issue. What I’m attempting to do is have a single table with all the achievements and their base information (name and description). Using that table and polymorphic associations, I want to link to other tables that will contain the criteria for completing that achievement, e.g. the WeightAchievement table will have the weight required and exercise id. Then, a user’s progress will be stored in a UserProgress model, where it links to the actual Achievement (as opposed to WeightAchievement).

The reason I need the criteria in separate tables is because the criteria will vary wildly between different types of achievements and will be added dynamically afterwards, which is why I’m not creating a separate model for each achievement.

Does this even make sense? Should I just merge the Achievement table with the specific type of achievement like WeightAchievement (so the table is name, description, weight_required, exercise_id), then when a user queries the achievements, in my code I simply search all the achievements? (e.g. WeightAchievement, EnduranceAchievement, RepAchievement, etc)

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.