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

You can find the interface for the AggregateReflection and AssociationReflection classes in the abstract MacroReflection class.

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

Source: show 

# File rails/activerecord/lib/active_record/reflection.rb, line 13
def create_reflection(macro, name, options, active_record)
  case macro
    when :has_many, :belongs_to, :has_one, :has_and_belongs_to_many
      reflection = AssociationReflection.new(macro, name, options, active_record)
    when :composed_of
      reflection = AggregateReflection.new(macro, name, options, active_record)
  end
  write_inheritable_hash :reflections, name => reflection
  reflection
end
reflect_on_aggregation(aggregation)

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

Account.reflect_on_aggregation(:balance) # returns the balance AggregateReflection

Source: show 

# File rails/activerecord/lib/active_record/reflection.rb, line 43
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 

# File rails/activerecord/lib/active_record/reflection.rb, line 35
def reflect_on_all_aggregations
  reflections.values.select { |reflection| reflection.is_a?(AggregateReflection) }
end
reflect_on_all_associations(macro = nil)

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

# File rails/activerecord/lib/active_record/reflection.rb, line 54
def reflect_on_all_associations(macro = nil)
  association_reflections = reflections.values.select { |reflection| reflection.is_a?(AssociationReflection) }
  macro ? association_reflections.select { |reflection| reflection.macro == macro } : association_reflections
end
reflect_on_association(association)

Returns the AssociationReflection object for the named aggregation (use the symbol). Example:

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

Source: show 

# File rails/activerecord/lib/active_record/reflection.rb, line 64
def reflect_on_association(association)
  reflections[association].is_a?(AssociationReflection) ? reflections[association] : nil
end
reflections()

Returns a hash containing all AssociationReflection objects for the current class Example:

Invoice.reflections
Account.reflections

Source: show 

# File rails/activerecord/lib/active_record/reflection.rb, line 30
def reflections
  read_inheritable_attribute(:reflections) || write_inheritable_attribute(:reflections, {})
end