Improve the performance of reading attributes

We added a comparison to "id", and call to `self.class.primary_key` a
*lot*. We also have performance hits from `&block` all over the place.
We skip the check in a new method, in order to avoid breaking the
behavior of `read_attribute`

3 files changed, 12 insertions, 6 deletions

diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 2f02738f6d..9ba46ec4c7 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -110,7 +110,7 @@ module ActiveRecord
         if attribute_changed?(attr)
           changed_attributes[attr]
         else
-          clone_attribute_value(:read_attribute, attr)
+          clone_attribute_value(:_read_attribute, attr)
         end
       end
 
diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index 104d84a1f8..c28374e4ab 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -17,7 +17,7 @@ module ActiveRecord
       def id
         if pk = self.class.primary_key
           sync_with_transaction_state
-          read_attribute(pk)
+          _read_attribute(pk)
         end
       end
 
diff --git a/activerecord/lib/active_record/attribute_methods/read.rb b/activerecord/lib/active_record/attribute_methods/read.rb
index 55beb309ca..20f0936e52 100644
--- a/activerecord/lib/active_record/attribute_methods/read.rb
+++ b/activerecord/lib/active_record/attribute_methods/read.rb
@@ -27,7 +27,7 @@ module ActiveRecord
           <<-EOMETHOD
           def #{method_name}
             name = ::ActiveRecord::AttributeMethods::AttrNames::ATTR_#{const_name}
-            read_attribute(name) { |n| missing_attribute(n, caller) }
+            _read_attribute(name) { |n| missing_attribute(n, caller) }
           end
           EOMETHOD
         end
@@ -64,7 +64,7 @@ module ActiveRecord
             generated_attribute_methods.module_eval <<-STR, __FILE__, __LINE__ + 1
               def #{temp_method}
                 name = ::ActiveRecord::AttributeMethods::AttrNames::ATTR_#{safe_name}
-                read_attribute(name) { |n| missing_attribute(n, caller) }
+                _read_attribute(name) { |n| missing_attribute(n, caller) }
               end
             STR
 
@@ -84,13 +84,19 @@ module ActiveRecord
       def read_attribute(attr_name, &block)
         name = attr_name.to_s
         name = self.class.primary_key if name == ID
-        @attributes.fetch_value(name, &block)
+        _read_attribute(name, &block)
+      end
+
+      # This method exists to avoid the expensive primary_key check internally, without
+      # breaking compatibility with the read_attribute API
+      def _read_attribute(attr_name) # :nodoc:
+        @attributes.fetch_value(attr_name.to_s) { |n| yield n if block_given? }
       end
 
       private
 
       def attribute(attribute_name)
-        read_attribute(attribute_name)
+        _read_attribute(attribute_name)
       end
     end
   end