From ede244e2367c7988fa02f5e3b956d88211bb5ade Mon Sep 17 00:00:00 2001
From: Sean Griffin <sean@seantheprogrammer.com>
Date: Fri, 2 Oct 2015 09:35:30 -0400
Subject: Build the `AttributeMutationTracker` lazily

For reads, we never need to construct this object. The double `defined?`
check is to avoid errors in tests
---
 .../lib/active_record/attribute_methods/dirty.rb   | 28 ++++++++++++----------
 activerecord/lib/active_record/persistence.rb      |  2 +-
 2 files changed, 16 insertions(+), 14 deletions(-)

(limited to 'activerecord/lib')

diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 17ec4c2252..e8a782ed13 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -35,27 +35,22 @@ module ActiveRecord
       # <tt>reload</tt> the record and clears changed attributes.
       def reload(*)
         super.tap do
-          @mutation_tracker = AttributeMutationTracker.new(@attributes)
+          @mutation_tracker = nil
           @previous_mutation_tracker = nil
           @changed_attributes = HashWithIndifferentAccess.new
         end
       end
 
-      def init_internals
-        super
-        @mutation_tracker = AttributeMutationTracker.new(@attributes)
-      end
-
       def initialize_dup(other) # :nodoc:
         super
         @attributes = self.class._default_attributes.map do |attr|
           attr.with_value_from_user(@attributes.fetch_value(attr.name))
         end
-        @mutation_tracker = AttributeMutationTracker.new(@attributes)
+        @mutation_tracker = nil
       end
 
       def changes_applied
-        @previous_mutation_tracker = @mutation_tracker
+        @previous_mutation_tracker = mutation_tracker
         @changed_attributes = HashWithIndifferentAccess.new
         store_original_attributes
       end
@@ -85,7 +80,7 @@ module ActiveRecord
         if defined?(@cached_changed_attributes)
           @cached_changed_attributes
         else
-          super.reverse_merge(@mutation_tracker.changed_values).freeze
+          super.reverse_merge(mutation_tracker.changed_values).freeze
         end
       end
 
@@ -100,17 +95,24 @@ module ActiveRecord
       end
 
       def attribute_changed_in_place?(attr_name)
-        @mutation_tracker.changed_in_place?(attr_name)
+        mutation_tracker.changed_in_place?(attr_name)
       end
 
       private
 
+      def mutation_tracker
+        unless defined?(@mutation_tracker)
+          @mutation_tracker = nil
+        end
+        @mutation_tracker ||= AttributeMutationTracker.new(@attributes)
+      end
+
       def changes_include?(attr_name)
-        super || @mutation_tracker.changed?(attr_name)
+        super || mutation_tracker.changed?(attr_name)
       end
 
       def clear_attribute_change(attr_name)
-        @mutation_tracker.forget_change(attr_name)
+        mutation_tracker.forget_change(attr_name)
       end
 
       def _update_record(*)
@@ -127,7 +129,7 @@ module ActiveRecord
 
       def store_original_attributes
         @attributes = @attributes.map(&:forgetting_assignment)
-        @mutation_tracker = AttributeMutationTracker.new(@attributes)
+        @mutation_tracker = nil
       end
 
       def previous_mutation_tracker
diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index 7b53f6e5a0..3f02f73a5a 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -211,7 +211,7 @@ module ActiveRecord
     def becomes(klass)
       became = klass.new
       became.instance_variable_set("@attributes", @attributes)
-      became.instance_variable_set("@mutation_tracker", @mutation_tracker)
+      became.instance_variable_set("@mutation_tracker", @mutation_tracker) if defined?(@mutation_tracker)
       became.instance_variable_set("@changed_attributes", attributes_changed_by_setter)
       became.instance_variable_set("@new_record", new_record?)
       became.instance_variable_set("@destroyed", destroyed?)
-- 
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