Reflection enables the ability to examine the associations and aggregations of Active Record classes and objects. This information, for example, can 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
R
Instance Public methods
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 66
def reflect_on_aggregation(aggregation)
  aggregate_reflections[aggregation.to_s]
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 58
def reflect_on_all_aggregations
  aggregate_reflections.values
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 103
def reflect_on_all_associations(macro = nil)
  association_reflections = reflections.values
  association_reflections.select! { |reflection| reflection.macro == macro } if 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 123
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 114
def reflect_on_association(association)
  reflections[association.to_s]
end
reflections()

Returns a Hash of name of the reflection as the key and an AssociationReflection as the value.

Account.reflections # => {"balance" => AggregateReflection}

Source: show | on GitHub 

# File activerecord/lib/active_record/reflection.rb, line 74
def reflections
  @__reflections ||= begin
    ref = {}

    _reflections.each do |name, reflection|
      parent_reflection = reflection.parent_reflection

      if parent_reflection
        parent_name = parent_reflection.name
        ref[parent_name.to_s] = parent_reflection
      else
        ref[name] = reflection
      end
    end

    ref
  end
end