module HasHistory
  # TODO (uwe)(2007-10-31): Allow different strategies: timestamped_master, separate_history_table, separate_history_table_with_last
  
  # Adds a trigger to the model that saves a copy of the model in a history table.
  #
  # The default history model is called <ModelName>History, but can be overridden by the "class_name" option.
  #
  # All columns are copied unless excluded by the :except or :only options.
  #
  # The created_at field is never changed in the original model, and is therefore omitted in the history model.
  #
  # The updated_at column should preserve the original timestamps but changes meaning in the history table.  It therefore is renamed to "valid_from".
  #
  # The original row is references by a foreign key column with default name "<lower case model name>_id".
  # This can be overridden by the :foreign_key option.
  #
  # You can automagically generate the history model class at runtime by adding the :generate_model => true option.
  # In that case, the following option is also available:
  #
  # * :allow_updates
  #
  # See HasHistory#acts_as_history for a description of this option.
  #
  # examples:
  #
  #   class Person < ActiveRecord::Base
  #     has_history
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :class_name => 'PersonHistory'
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :except => :birthdate
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :except => [:birthdate, :social_security_no]
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :only => :weight
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :only => [:height, :weight]
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :foreign_key => :person_id
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :generate_model => true
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :generate_model => true, :allow_updates => true
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :generate_model => true, :allow_updates => true
  #   end
  #
  def has_history options = {}
    options[:only] = [options[:only]] if options[:only].is_a? Symbol
    options[:except] = [options[:except]] if options[:except].is_a? Symbol
    history_model_class_name = (options[:class_name] && options[:class_name].to_s) || "#{name}History"
    
    if options[:generate_model]
      Object::class_eval <<-EOF
class #{history_model_class_name} < ActiveRecord::Base
  acts_as_history :class_name => :#{table_name.singularize}, :allow_updates => #{options[:allow_updates].inspect}
end
EOF
    end
    
    history_model = history_model_class_name.constantize
    foreign_key = options[:foreign_key] || "#{table_name.singularize}_id".to_sym
    
    before_update do |record|
      old_values = find(record.id).attributes.symbolize_keys
      new_values = record.attributes.symbolize_keys
      values = {foreign_key => record.id}.update(old_values)
      old_values.delete(:updated_at)
      new_values.delete(:updated_at)
      unless new_values == old_values
        values[:valid_from] = values.delete(:updated_at) if values[:updated_at]
        values.delete(:id)
        values.delete_if {|k,v| not history_model.column_names.include? k.to_s}
        values.delete_if {|k,v| not options[:only].include? k} if options[:only]
        values.delete_if {|k,v| options[:except].include? k} if options[:except]
        history_model.create!(values)
      end
    end
    if options[:order] 
      order = options[:order]
    elsif history_model.columns.find {|c| c.name == 'valid_from'} 
      order = 'valid_from DESC'
    else
      order = 'id'
    end
    has_many history_model.table_name.pluralize.to_sym, :dependent => :destroy, :foreign_key => foreign_key, :order => order
  end
  
  # Adds a belongs_to relationship to a master model and blocks updates and destroy actions on this class.
  #
  # If the class name of the history model ends with "History", the default master model class name is automatically calculated (ie. PersonHistory => Person).
  # If the history class name does not end in "History", or you want to use another class name, you must override it with the :class_name option.
  #
  # Normally the history model is blocked for updates, but updates can be enabled by the :allow_updates option.
  #
  # The original row is references by a foreign key column with default name "<lower case model name>_id".
  # This can be overridden by the "foreign_key" option.
  #
  # If you would like to inject the update trigger in the master model from the history model, use the :inject_trigger => true option.
  # If you do this, these additional options are available:
  #
  # * :only
  # * :except
  #
  # See HasHistory#has_history for a description of these options.
  #
  # examples:
  #
  #   class PersonHistory < ActiveRecord::Base
  #     acts_as_history
  #   end
  #
  #   class PersonRecords < ActiveRecord::Base
  #     acts_as_history :class_name => 'Person'
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     acts_as_history :allow_updates => true
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     acts_as_history :inject_trigger => true, :except => :birthdate
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     acts_as_history :inject_trigger => true, :except => [:birthdate, :social_security_no]
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     acts_as_history :inject_trigger => true, :only => :weight
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     acts_as_history :inject_trigger => true, :only => [:height, :weight]
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :foreign_key => :person_id
  #   end
  #
  #   class Person < ActiveRecord::Base
  #     has_history :generate_model => true
  #   end
  #
  def acts_as_history options = {}
    model_name = options[:class_name] || (name =~ /(.*)History/ && $1.constantize.table_name.singularize.to_sym)
    raise "Master model class name '#{$1}' is missing and cannot be deduced." if model_name.nil?
    belongs_to model_name
    unless options[:allow_updates]
      validate_on_update do |record|
        record.errors.add "History:", "You may not change history!"
      end
      before_destroy do |record|
        record.errors.add :id, "You may not change history!"
        false
      end
    end
  end
  
end