inheritance.rb

require 'active_support/core_ext/hash/indifferent_access'

module ActiveRecord
  # == Single table inheritance
  #
  # Active Record allows inheritance by storing the name of the class in a column that by
  # default is named "type" (can be changed by overwriting <tt>Base.inheritance_column</tt>).
  # This means that an inheritance looking like this:
  #
  #   class Company < ActiveRecord::Base; end
  #   class Firm < Company; end
  #   class Client < Company; end
  #   class PriorityClient < Client; end
  #
  # When you do <tt>Firm.create(name: "37signals")</tt>, this record will be saved in
  # the companies table with type = "Firm". You can then fetch this row again using
  # <tt>Company.where(name: '37signals').first</tt> and it will return a Firm object.
  #
  # Be aware that because the type column is an attribute on the record every new
  # subclass will instantly be marked as dirty and the type column will be included
  # in the list of changed attributes on the record. This is different from non
  # STI classes:
  #
  #   Company.new.changed? # => false
  #   Firm.new.changed?    # => true
  #   Firm.new.changes     # => {"type"=>["","Firm"]}
  #
  # If you don't have a type column defined in your table, single-table inheritance won't
  # be triggered. In that case, it'll work just like normal subclasses with no special magic
  # for differentiating between them or reloading the right type with find.
  #
  # Note, all the attributes for all the cases are kept in the same table. Read more:
  # http://www.martinfowler.com/eaaCatalog/singleTableInheritance.html
  #
  module Inheritance
    extend ActiveSupport::Concern

    included do
      # Determines whether to store the full constant name including namespace when using STI.
      class_attribute :store_full_sti_class, instance_writer: false
      self.store_full_sti_class = true
    end

    module ClassMethods
      # Determines if one of the attributes passed in is the inheritance column,
      # and if the inheritance column is attr accessible, it initializes an
      # instance of the given subclass instead of the base class.
      def new(*args, &block)
        if abstract_class? || self == Base
          raise NotImplementedError, "#{self} is an abstract class and cannot be instantiated."
        end

        attrs = args.first
        if subclass_from_attributes?(attrs)
          subclass = subclass_from_attributes(attrs)
        end

        if subclass
          subclass.new(*args, &block)
        else
          super
        end
      end

      # Returns +true+ if this does not need STI type condition. Returns
      # +false+ if STI type condition needs to be applied.
      def descends_from_active_record?
        if self == Base
          false
        elsif superclass.abstract_class?
          superclass.descends_from_active_record?
        else
          superclass == Base || !columns_hash.include?(inheritance_column)
        end
      end

      def finder_needs_type_condition? #:nodoc:
        # This is like this because benchmarking justifies the strange :false stuff
        :true == (@finder_needs_type_condition ||= descends_from_active_record? ? :false : :true)
      end

      def symbolized_base_class
        ActiveSupport::Deprecation.warn('`ActiveRecord::Base.symbolized_base_class` is deprecated and will be removed without replacement.')
        @symbolized_base_class ||= base_class.to_s.to_sym
      end

      def symbolized_sti_name
        ActiveSupport::Deprecation.warn('`ActiveRecord::Base.symbolized_sti_name` is deprecated and will be removed without replacement.')
        @symbolized_sti_name ||= sti_name.present? ? sti_name.to_sym : symbolized_base_class
      end

      # Returns the class descending directly from ActiveRecord::Base, or
      # an abstract class, if any, in the inheritance hierarchy.
      #
      # If A extends AR::Base, A.base_class will return A. If B descends from A
      # through some arbitrarily deep hierarchy, B.base_class will return A.
      #
      # If B < A and C < B and if A is an abstract_class then both B.base_class
      # and C.base_class would return B as the answer since A is an abstract_class.
      def base_class
        unless self < Base
          raise ActiveRecordError, "#{name} doesn't belong in a hierarchy descending from ActiveRecord"
        end

        if superclass == Base || superclass.abstract_class?
          self
        else
          superclass.base_class
        end
      end

      # Set this to true if this is an abstract class (see <tt>abstract_class?</tt>).
      # If you are using inheritance with ActiveRecord and don't want child classes
      # to utilize the implied STI table name of the parent class, this will need to be true.
      # For example, given the following:
      #
      #   class SuperClass < ActiveRecord::Base
      #     self.abstract_class = true
      #   end
      #   class Child < SuperClass
      #     self.table_name = 'the_table_i_really_want'
      #   end
      #
      #
      # <tt>self.abstract_class = true</tt> is required to make <tt>Child<.find,.create, or any Arel method></tt> use <tt>the_table_i_really_want</tt> instead of a table called <tt>super_classes</tt>
      #
      attr_accessor :abstract_class

      # Returns whether this class is an abstract class or not.
      def abstract_class?
        defined?(@abstract_class) && @abstract_class == true
      end

      def sti_name
        store_full_sti_class ? name : name.demodulize
      end

      protected

      # Returns the class type of the record using the current module as a prefix. So descendants of
      # MyApp::Business::Account would appear as MyApp::Business::AccountSubclass.
      def compute_type(type_name)
        if type_name.match(/^::/)
          # If the type is prefixed with a scope operator then we assume that
          # the type_name is an absolute reference.
          ActiveSupport::Dependencies.constantize(type_name)
        else
          # Build a list of candidates to search for
          candidates = []
          name.scan(/::|$/) { candidates.unshift "#{$`}::#{type_name}" }
          candidates << type_name

          candidates.each do |candidate|
            constant = ActiveSupport::Dependencies.safe_constantize(candidate)
            return constant if candidate == constant.to_s
          end

          raise NameError.new("uninitialized constant #{candidates.first}", candidates.first)
        end
      end

      private

      # Called by +instantiate+ to decide which class to use for a new
      # record instance. For single-table inheritance, we check the record
      # for a +type+ column and return the corresponding class.
      def discriminate_class_for_record(record)
        if using_single_table_inheritance?(record)
          find_sti_class(record[inheritance_column])
        else
          super
        end
      end

      def using_single_table_inheritance?(record)
        record[inheritance_column].present? && columns_hash.include?(inheritance_column)
      end

      def find_sti_class(type_name)
        if store_full_sti_class
          ActiveSupport::Dependencies.constantize(type_name)
        else
          compute_type(type_name)
        end
      rescue NameError
        raise SubclassNotFound,
          "The single-table inheritance mechanism failed to locate the subclass: '#{type_name}'. " +
          "This error is raised because the column '#{inheritance_column}' is reserved for storing the class in case of inheritance. " +
          "Please rename this column if you didn't intend it to be used for storing the inheritance class " +
          "or overwrite #{name}.inheritance_column to use another column for that information."
      end

      def type_condition(table = arel_table)
        sti_column = table[inheritance_column]
        sti_names  = ([self] + descendants).map(&:sti_name)

        sti_column.in(sti_names)
      end

      # Detect the subclass from the inheritance column of attrs. If the inheritance column value
      # is not self or a valid subclass, raises ActiveRecord::SubclassNotFound
      # If this is a StrongParameters hash, and access to inheritance_column is not permitted,
      # this will ignore the inheritance column and return nil
      def subclass_from_attributes?(attrs)
        columns_hash.include?(inheritance_column) && attrs.is_a?(Hash)
      end

      def subclass_from_attributes(attrs)
        subclass_name = attrs.with_indifferent_access[inheritance_column]

        if subclass_name.present? && subclass_name != self.name
          subclass = subclass_name.safe_constantize

          unless descendants.include?(subclass)
            raise ActiveRecord::SubclassNotFound.new("Invalid single-table inheritance type: #{subclass_name} is not a subclass of #{name}")
          end

          subclass
        end
      end
    end

    def initialize_dup(other)
      super
      ensure_proper_type
    end

    private

    def initialize_internals_callback
      super
      ensure_proper_type
    end

    # Sets the attribute used for single table inheritance to this class name if this is not the
    # ActiveRecord::Base descendant.
    # Considering the hierarchy Reply < Message < ActiveRecord::Base, this makes it possible to
    # do Reply.new without having to set <tt>Reply[Reply.inheritance_column] = "Reply"</tt> yourself.
    # No such attribute would be set for objects of the Message class in that example.
    def ensure_proper_type
      klass = self.class
      if klass.finder_needs_type_condition?
        write_attribute(klass.inheritance_column, klass.sti_name)
      end
    end
  end
end