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
Public Instance methods
create_reflection(macro, name, options, active_record)
    # File vendor/rails/activerecord/lib/active_record/reflection.rb, line 13
13:       def create_reflection(macro, name, options, active_record)
14:         case macro
15:           when :has_many, :belongs_to, :has_one, :has_and_belongs_to_many
16:             reflection = AssociationReflection.new(macro, name, options, active_record)
17:           when :composed_of
18:             reflection = AggregateReflection.new(macro, name, options, active_record)
19:         end
20:         write_inheritable_hash :reflections, name => reflection
21:         reflection
22:       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
    # File vendor/rails/activerecord/lib/active_record/reflection.rb, line 35
35:       def reflect_on_aggregation(aggregation)
36:         reflections[aggregation].is_a?(AggregateReflection) ? reflections[aggregation] : nil
37:       end
reflect_on_all_aggregations()

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

    # File vendor/rails/activerecord/lib/active_record/reflection.rb, line 29
29:       def reflect_on_all_aggregations
30:         reflections.values.select { |reflection| reflection.is_a?(AggregateReflection) }
31:       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

    # File vendor/rails/activerecord/lib/active_record/reflection.rb, line 43
43:       def reflect_on_all_associations(macro = nil)
44:         association_reflections = reflections.values.select { |reflection| reflection.is_a?(AssociationReflection) }
45:         macro ? association_reflections.select { |reflection| reflection.macro == macro } : association_reflections
46:       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
    # File vendor/rails/activerecord/lib/active_record/reflection.rb, line 51
51:       def reflect_on_association(association)
52:         reflections[association].is_a?(AssociationReflection) ? reflections[association] : nil
53:       end
reflections()
    # File vendor/rails/activerecord/lib/active_record/reflection.rb, line 24
24:       def reflections
25:         read_inheritable_attribute(:reflections) or write_inheritable_attribute(:reflections, {})
26:       end