AR Criteria

criteria.rb

class Invoice < ActiveRecord::Base
  has_many :line_items
end

class LineItemTemplate < ActiveRecord::Base
  has_many :criteria
end

class Criteria < ActiveRecord::Base
  belongs_to :line_item_template
  # would have attributes something like:
  #   source_field (attribute on invoice to examine e.g. :total)
  #   comparison   (equal, less_than, greater_than)
  #   value        (number or expression)
end

# Here's an example of ANDed criteria
t = LineItemTemplate.create(:description  => "overweight shipping charge to Canada", :require_all_criteria => true)
t.criteria.create(
  :source_field => "total_weight",
  :comparison   => "greater_than",
  :value        => "100"
)
t.criteria.create(
  :source_field => "destination_country",
  :comparison   => "equal",
  :value        => "CA"
)

# If I was to build this, I'd probably make criteria acts_as_nested_set to model arbitrary  (condition AND (condition OR condition)) stuff...