Reflection enables to interrogate Active Record classes and objects about their associations and aggregations. This information can, for example, be used in a form builder that takes an Active Record object and creates input fields for all of the attributes depending on their type and displays the associations to other objects.

MacroReflection class has info for AggregateReflection and AssociationReflection classes.

Methods
C
R
Instance Public methods
create_reflection(macro, name, options, active_record)

Source: show | on GitHub 

# File activerecord/lib/active_record/reflection.rb, line 23
def create_reflection(macro, name, options, active_record)
  case macro
    when :has_many, :belongs_to, :has_one, :has_and_belongs_to_many
      klass = options[:through] ? ThroughReflection : AssociationReflection
      reflection = klass.new(macro, name, options, active_record)
    when :composed_of
      reflection = AggregateReflection.new(macro, name, options, active_record)
  end

  self.reflections = self.reflections.merge(name => reflection)
  reflection
end
reflect_on_aggregation(aggregation)

Returns the AggregateReflection object for the named aggregation (use the symbol).

Account.reflect_on_aggregation(:balance) # => the balance AggregateReflection

Source: show | on GitHub 

# File activerecord/lib/active_record/reflection.rb, line 45
def reflect_on_aggregation(aggregation)
  reflections[aggregation].is_a?(AggregateReflection) ? reflections[aggregation] : nil
end
reflect_on_all_aggregations()

Returns an array of AggregateReflection objects for all the aggregations in the class.

Source: show | on GitHub 

# File activerecord/lib/active_record/reflection.rb, line 37
def reflect_on_all_aggregations
  reflections.values.grep(AggregateReflection)
end
reflect_on_all_associations(macro = nil)

Returns an array of AssociationReflection objects for all the associations in the class. If you only want to reflect on a certain association type, pass in the symbol (:has_many, :has_one, :belongs_to) as the first parameter.

Example:

Account.reflect_on_all_associations             # returns an array of all associations
Account.reflect_on_all_associations(:has_many)  # returns an array of all has_many associations

Source: show | on GitHub 

# File activerecord/lib/active_record/reflection.rb, line 59
def reflect_on_all_associations(macro = nil)
  association_reflections = reflections.values.grep(AssociationReflection)
  macro ? association_reflections.select { |reflection| reflection.macro == macro } : association_reflections
end
reflect_on_all_autosave_associations()

Returns an array of AssociationReflection objects for all associations which have :autosave enabled.

Source: show | on GitHub 

# File activerecord/lib/active_record/reflection.rb, line 74
def reflect_on_all_autosave_associations
  reflections.values.select { |reflection| reflection.options[:autosave] }
end
reflect_on_association(association)

Returns the AssociationReflection object for the association (use the symbol).

Account.reflect_on_association(:owner)             # returns the owner AssociationReflection
Invoice.reflect_on_association(:line_items).macro  # returns :has_many

Source: show | on GitHub 

# File activerecord/lib/active_record/reflection.rb, line 69
def reflect_on_association(association)
  reflections[association].is_a?(AssociationReflection) ? reflections[association] : nil
end