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
# Single Table Inheritance(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.
# This is true, by default.
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 has_attribute?(inheritance_column)
subclass = subclass_from_attributes(attrs)
if subclass.nil? && base_class == self
subclass = subclass_from_attributes(column_defaults)
end
end
if subclass && subclass != self
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
# Returns the class descending directly from ActiveRecord::Base, or
# an abstract class, if any, in the inheritance hierarchy.
#
# If A extends ActiveRecord::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? && has_attribute?(inheritance_column)
end
def find_sti_class(type_name)
type_name = base_class.type_for_attribute(inheritance_column).cast(type_name)
subclass = begin
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
unless subclass == self || descendants.include?(subclass)
raise SubclassNotFound, "Invalid single-table inheritance type: #{subclass.name} is not a subclass of #{name}"
end
subclass
end
def type_condition(table = arel_table)
sti_column = arel_attribute(inheritance_column, table)
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
def subclass_from_attributes(attrs)
attrs = attrs.to_h if attrs.respond_to?(:permitted?)
if attrs.is_a?(Hash)
subclass_name = attrs.with_indifferent_access[inheritance_column]
if subclass_name.present?
find_sti_class(subclass_name)
end
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
