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 Base.inheritance_column). # 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 Firm.create(name: "37signals"), this record will be saved in # the companies table with type = "Firm". You can then fetch this row again using # Company.where(name: '37signals').first 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, default: 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 abstract_class?). # 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 # # # self.abstract_class = true is required to make Child<.find,.create, or any Arel method> use the_table_i_really_want instead of a table called super_classes # 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 def inherited(subclass) subclass.instance_variable_set(:@_type_candidates_cache, Concurrent::Map.new) super 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.start_with?("::".freeze) # 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 type_candidate = @_type_candidates_cache[type_name] if type_candidate && type_constant = ActiveSupport::Dependencies.safe_constantize(type_candidate) return type_constant end # 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) if candidate == constant.to_s @_type_candidates_cache[type_name] = candidate return constant end 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[inheritance_column] || attrs[inheritance_column.to_sym] 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 Reply[Reply.inheritance_column] = "Reply" 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