Override _read_attribute in Dirty plugin

This is required due to a fix in AR 5.2:

rails/rails#28661

Previously, `__send__` was used to fetch the value of the attribute
before changes, which worked fine with locale accessors like
`title_en`. However, with the fix above, `@attributes` is now used,
which does not contain translated attributes (let alone locale
accessors). Since in general, locale accessors are an open-ended set, we
can't treat them the way normal attributes are treated, so we resort to
overriding `_read_attribute` to check if the attribute that has changed
is one that Mobility changed, and if so use `__send__`. This fixes the
issue, although overriding a private method is not ideal.

lib/mobility/plugins/active_model/dirty.rb

@@ -32,6 +32,7 @@ def write(locale, value, options = {})
           if model.changed_attributes.has_key?(locale_accessor) && model.changed_attributes[locale_accessor] == value
             model.send(:attributes_changed_by_setter).except!(locale_accessor)
           elsif read(locale, options.merge(fallback: false)) != value
+            model.send(:mobility_changed_attributes) << locale_accessor
             model.send(:attribute_will_change!, locale_accessor)
           end
           super
@@ -63,6 +64,10 @@ def initialize(*attribute_names)
             private :restore_attribute!
           end
 
+          def included(model_class)
+            model_class.include ChangedAttributes
+          end
+
           private
 
           # Get method suffixes. Creating an object just to get the list of
@@ -74,6 +79,19 @@ def method_suffixes
                 include ::ActiveModel::Dirty
               end.attribute_method_matchers.map(&:suffix).select { |m| m =~ /\A_/ }
           end
+
+          # Tracks which translated attributes have been changed, separate from
+          # the default tracking of changes in ActiveModel/ActiveRecord Dirty.
+          # This is required in order for the Mobility ActiveRecord Dirty
+          # plugin to correctly read the value of locale accessors like
+          # +title_en+ in dirty tracking.
+          module ChangedAttributes
+            private
+
+            def mobility_changed_attributes
+              @mobility_changed_attributes ||= Set.new
+            end
+          end
         end
       end
     end

lib/mobility/plugins/active_record/dirty.rb

@@ -34,23 +34,6 @@ def initialize(*attribute_names)
             define_attribute_methods if ::ActiveRecord::VERSION::STRING >= '5.1'
           end
 
-          # Overrides +ActiveRecord::AttributeMethods::ClassMethods#has_attribute+ to treat fallthrough attribute methods
-          # just like "real" attribute methods.
-          #
-          # @note Patching +has_attribute?+ is necessary as of AR 5.1 due to this commit[https://github.com/rails/rails/commit/4fed08fa787a316fa51f14baca9eae11913f5050].
-          #   (I have voiced my opposition to this change here[https://github.com/rails/rails/pull/27963#issuecomment-310092787]).
-          # @param [Attributes] attributes
-          def included(model_class)
-            names = @attribute_names
-            method_name_regex = /\A(#{names.join('|'.freeze)})_([a-z]{2}(_[a-z]{2})?)(=?|\??)\z/.freeze
-            has_attribute = Module.new do
-              define_method :has_attribute? do |attr_name|
-                super(attr_name) || !!method_name_regex.match(attr_name)
-              end
-            end
-            model_class.extend has_attribute
-          end
-
           private
 
           def define_method_overrides
@@ -94,6 +77,46 @@ def define_attribute_methods
               alias_method :"#{name}_in_database", :"#{name}_was"
             end
           end
+
+          # Overrides +ActiveRecord::AttributeMethods::ClassMethods#has_attribute+ and
+          # +ActiveModel::AttributeMethods#_read_attribute+ to treat
+          # fallthrough attribute methods just like "real" attribute methods.
+          #
+          # @note Patching +has_attribute?+ is necessary as of AR 5.1 due to this commit[https://github.com/rails/rails/commit/4fed08fa787a316fa51f14baca9eae11913f5050].
+          #   (I have voiced my opposition to this change here[https://github.com/rails/rails/pull/27963#issuecomment-310092787]).
+          # @param [Attributes] attributes
+          def included(model_class)
+            super
+            names = @attribute_names
+            method_name_regex = /\A(#{names.join('|'.freeze)})_([a-z]{2}(_[a-z]{2})?)(=?|\??)\z/.freeze
+            has_attribute = Module.new do
+              define_method :has_attribute? do |attr_name|
+                super(attr_name) || !!method_name_regex.match(attr_name)
+              end
+            end
+            model_class.extend has_attribute
+            model_class.include ReadAttribute
+          end
+
+          # Overrides _read_attribute to correctly dispatch reads on translated
+          # attributes to their respective setters, rather than to
+          # +@attributes+, which would otherwise return +nil+.
+          #
+          # For background on why this is necessary, see:
+          # https://github.com/shioyama/mobility/issues/115
+          module ReadAttribute
+            # @note We first check if attributes has the key +attr+ to avoid
+            # doing any extra work in case this is a "normal" (non-translated)
+            # attribute.
+            def _read_attribute(attr, *args)
+              if @attributes.key?(attr)
+                super
+              else
+                mobility_changed_attributes.include?(attr) ? __send__(attr) : super
+              end
+            end
+            private :_read_attribute
+          end
         end
       end
     end