aboutsummaryrefslogtreecommitdiffstats
path: root/activerecord/lib/active_record/inheritance.rb
blob: 1753322274e00be35e6ce2829ddc2bbb8fe3e650 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
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, 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 <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

      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 <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