Goals are a high level concept--having a user set a goal should be validated in a way that keeps the model closed for modification, allows maximum flexibility in expressing goal validation per user, and keeps the data in the database rather than mixed throughout the application logic.

declarative_goal_specifications.rb

# - simplified
module Goals
  class WeightLossGoal < Goal

    db_identifier :weight_loss_goal

    validates_goal_with do |user, amount_weight|
      # user is the user we are validating against
      if amount_weight >= user.weight
        raise InvalidGoalError.new('cannot lose more than you weigh')
      end
    end

  end
end

# - overarching module controlling this
module Goals

  def self.load_goals
    Goal.registered_goals do |registered_goal|
      # make sure it is loaded into the database if it does not exist

      # then make sure goals not defined are set to disabled

      # to keep this fast, the ID should have an index and do a NOT IN
      # query to disable undefined goal types

      # NOTE: as conditions change, it is possible for invalid data to
      # be in the database...you just cannot update it with invalid data
    end
  end

end

module Goals
  class Goal
    @@registered_goals = []
    # on inherited, make sure the class is registered

    def self.registered_goals
      @@registered_goals
    end

    # declarative stuff here

  end

  class AmmountGoal < Goal

  end
end

module Goals
  class WeightLossGoal < AmmountGoal

    db_identifier :weight_loss_goal

    enabled # disabled would also work

    name 'Lose weight'

    description 'Lose a certain amount of weight'

    validates_goal_with do |user, amount_weight|
      # user is the user we are validating against
      if amount_weight >= user.weight
        raise InvalidGoalError.new('cannot lose more than you weigh')
      end
    end

  end
end

module Goals
  class WalkMoreGoal < AmmountGoal

    db_identifier :walk_more_goal

    enabled

    name 'Walk more'

    description 'A goal to walk more'

    task 'park at the far end of the parking lot'

    validates_goal_with do |user, amount_steps|
      if amount_steps > 120000
        raise InvalidGoalError.new('Too many steps')
      end
    end
  end
end

# - in initializers file

#Goal.load_goals # or Goal.load_goals do |config| end

# any goals that are in the db but not defined in here
# should be disabled in the database so that they cannot
# be used. this could also be pacakged as a separate gem
# that also includes a rails engine with necessary migrations
# for the database...
# the gem could also include an initializer with it

# - migrations to be done
# create goal tables
# id, goal_identifier, enabled
# index on goal_identifier

# - activerecord validator
# in Rails::Engine do: config.autoload_paths << File.expand_path("../app/validators", __FILE__)
# app/validators/goal_validator.rb
class GoalValidator < ActiveModel::Validator
  def validate(record)
    begin
      Goal.find(record.send(options[:goal_identifier])).validate(record.send(options[:user]), record.send(options[:amount]))
    rescue => error
      record.errors[:base] << error
    end
  end
end

# - activerecord validator (if goal not defined, no need to do any special processing)
# in Rails::Engine do: config.autoload_paths << File.expand_path("../app/validators", __FILE__)
# app/validators/goal_validator.rb
class GoalValidator < ActiveModel::Validator
  def validate(record)
    goal = Goal.find(record.send(options[:goal_identifier]))
    return if goal.nil?

    begin
      goal.validate(record.send(options[:user]), record.send(options[:amount]))
    rescue => error
      record.errors[:base] << error
    end
  end
end

# - in models, user_goals.rb
class UserGoal < ActiveRecord::Base
  validates_with GoalValidator, goal_identifier: :goal_identifier, user: :user, amount: :amount
end


# - use case
User.goals << UserGoal.new('WeightLossGoal', 12) # will run the custom validator
User.goals << UserGoal.new('StepsGoal', 122) # no custom validator will run...if we add one later it will automatically be run