PERF: 20% faster pk attribute access

I've realized that `user.id` is 20% slower than `user.name` in the
benchmark (https://github.com/rails/rails/pull/35987#issuecomment-483882480).

The reason that performance difference is that `self.class.primary_key`
method call is a bit slow.

Avoiding that method call will make almost attribute access faster and
`user.id` will be completely the same performance with `user.name`.

Before (02b5b8cb):

```
Warming up --------------------------------------
             user.id   140.535k i/100ms
          user['id']    96.549k i/100ms
           user.name   158.110k i/100ms
        user['name']    94.507k i/100ms
       user.changed?    19.003k i/100ms
 user.saved_changes?    25.404k i/100ms
Calculating -------------------------------------
             user.id      2.231M (± 0.9%) i/s -     11.243M in   5.040066s
          user['id']      1.310M (± 1.3%) i/s -      6.565M in   5.012607s
           user.name      2.683M (± 1.2%) i/s -     13.439M in   5.009392s
        user['name']      1.322M (± 0.9%) i/s -      6.615M in   5.003239s
       user.changed?    201.999k (±10.9%) i/s -      1.007M in   5.091195s
 user.saved_changes?    258.214k (±17.1%) i/s -      1.245M in   5.007421s
```

After (this change):

```
Warming up --------------------------------------
             user.id   158.364k i/100ms
          user['id']   106.412k i/100ms
           user.name   158.644k i/100ms
        user['name']   107.518k i/100ms
       user.changed?    19.082k i/100ms
 user.saved_changes?    24.886k i/100ms
Calculating -------------------------------------
             user.id      2.768M (± 1.1%) i/s -     13.936M in   5.034957s
          user['id']      1.507M (± 2.1%) i/s -      7.555M in   5.017211s
           user.name      2.727M (± 1.5%) i/s -     13.643M in   5.004766s
        user['name']      1.521M (± 1.3%) i/s -      7.634M in   5.018321s
       user.changed?    200.865k (±11.1%) i/s -    992.264k in   5.044868s
 user.saved_changes?    269.652k (±10.5%) i/s -      1.344M in   5.077972s
```

8 files changed, 22 insertions, 26 deletions

diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index 331813f338..220043c061 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -465,7 +465,7 @@ module ActiveRecord
       end
 
       def pk_attribute?(name)
-        name == self.class.primary_key
+        name == @primary_key
       end
   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 feaef72a30..b4f5e6e75a 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -16,34 +16,32 @@ module ActiveRecord
 
       # Returns the primary key column's value.
       def id
-        primary_key = self.class.primary_key
-        _read_attribute(primary_key) if primary_key
+        _read_attribute(@primary_key)
       end
 
       # Sets the primary key column's value.
       def id=(value)
-        primary_key = self.class.primary_key
-        _write_attribute(primary_key, value) if primary_key
+        _write_attribute(@primary_key, value)
       end
 
       # Queries the primary key column's value.
       def id?
-        query_attribute(self.class.primary_key)
+        query_attribute(@primary_key)
       end
 
       # Returns the primary key column's value before type cast.
       def id_before_type_cast
-        read_attribute_before_type_cast(self.class.primary_key)
+        read_attribute_before_type_cast(@primary_key)
       end
 
       # Returns the primary key column's previous value.
       def id_was
-        attribute_was(self.class.primary_key)
+        attribute_was(@primary_key)
       end
 
       # Returns the primary key column's value from the database.
       def id_in_database
-        attribute_in_database(self.class.primary_key)
+        attribute_in_database(@primary_key)
       end
 
       private
@@ -116,7 +114,7 @@ module ActiveRecord
           #
           #   Project.primary_key # => "foo_id"
           def primary_key=(value)
-            @primary_key        = value && value.to_s
+            @primary_key        = value && -value.to_s
             @quoted_primary_key = nil
             @attributes_builder = nil
           end
diff --git a/activerecord/lib/active_record/attribute_methods/read.rb b/activerecord/lib/active_record/attribute_methods/read.rb
index 409a150e56..c787fb6a94 100644
--- a/activerecord/lib/active_record/attribute_methods/read.rb
+++ b/activerecord/lib/active_record/attribute_methods/read.rb
@@ -31,8 +31,7 @@ module ActiveRecord
           name = self.class.attribute_alias(name)
         end
 
-        primary_key = self.class.primary_key
-        name = primary_key if name == "id" && primary_key
+        name = @primary_key if name == "id" && @primary_key
         _read_attribute(name, &block)
       end
 
diff --git a/activerecord/lib/active_record/attribute_methods/write.rb b/activerecord/lib/active_record/attribute_methods/write.rb
index 6a21643884..6a54bd2fc2 100644
--- a/activerecord/lib/active_record/attribute_methods/write.rb
+++ b/activerecord/lib/active_record/attribute_methods/write.rb
@@ -35,8 +35,7 @@ module ActiveRecord
           name = self.class.attribute_alias(name)
         end
 
-        primary_key = self.class.primary_key
-        name = primary_key if name == "id" && primary_key
+        name = @primary_key if name == "id" && @primary_key
         _write_attribute(name, value)
       end
 
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index b9046fcc1a..068ebf3c09 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -174,8 +174,7 @@ module ActiveRecord
 
         record = statement.execute([id], connection)&.first
         unless record
-          raise RecordNotFound.new("Couldn't find #{name} with '#{primary_key}'=#{id}",
-                                   name, primary_key, id)
+          raise RecordNotFound.new("Couldn't find #{name} with '#{key}'=#{id}", name, key, id)
         end
         record
       end
@@ -398,7 +397,7 @@ module ActiveRecord
     ##
     def initialize_dup(other) # :nodoc:
       @attributes = @attributes.deep_dup
-      @attributes.reset(self.class.primary_key)
+      @attributes.reset(@primary_key)
 
       _run_initialize_callbacks
 
@@ -570,6 +569,7 @@ module ActiveRecord
       end
 
       def init_internals
+        @primary_key              = self.class.primary_key
         @readonly                 = false
         @destroyed                = false
         @marked_for_destruction   = false
diff --git a/activerecord/lib/active_record/locking/optimistic.rb b/activerecord/lib/active_record/locking/optimistic.rb
index b7eecda59e..6711ee9bf4 100644
--- a/activerecord/lib/active_record/locking/optimistic.rb
+++ b/activerecord/lib/active_record/locking/optimistic.rb
@@ -87,7 +87,7 @@ module ActiveRecord
 
             affected_rows = self.class._update_record(
               attributes_with_values(attribute_names),
-              self.class.primary_key => id_in_database,
+              @primary_key => id_in_database,
               locking_column => previous_lock_value
             )
 
@@ -110,7 +110,7 @@ module ActiveRecord
           locking_column = self.class.locking_column
 
           affected_rows = self.class._delete_record(
-            self.class.primary_key => id_in_database,
+            @primary_key => id_in_database,
             locking_column => read_attribute_before_type_cast(locking_column)
           )
 
diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index bd572486c8..1ff7ee4d89 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -353,6 +353,7 @@ module ActiveRecord
       end
 
       def _insert_record(values) # :nodoc:
+        primary_key = self.primary_key
         primary_key_value = nil
 
         if primary_key && Hash === values
@@ -674,7 +675,7 @@ module ActiveRecord
 
       affected_rows = self.class._update_record(
         attributes,
-        self.class.primary_key => id_in_database
+        @primary_key => id_in_database
       )
 
       affected_rows == 1
@@ -874,7 +875,7 @@ module ActiveRecord
     end
 
     def _delete_row
-      self.class._delete_record(self.class.primary_key => id_in_database)
+      self.class._delete_record(@primary_key => id_in_database)
     end
 
     def _touch_row(attribute_names, time)
@@ -890,7 +891,7 @@ module ActiveRecord
     def _update_row(attribute_names, attempted_action = "update")
       self.class._update_record(
         attributes_with_values(attribute_names),
-        self.class.primary_key => id_in_database
+        @primary_key => id_in_database
       )
     end
 
@@ -928,7 +929,7 @@ module ActiveRecord
         attributes_with_values(attribute_names)
       )
 
-      self.id ||= new_id if self.class.primary_key
+      self.id ||= new_id if @primary_key
 
       @new_record = false
 
diff --git a/activerecord/lib/active_record/transactions.rb b/activerecord/lib/active_record/transactions.rb
index 9f52734e00..49a8206c84 100644
--- a/activerecord/lib/active_record/transactions.rb
+++ b/activerecord/lib/active_record/transactions.rb
@@ -432,9 +432,8 @@ module ActiveRecord
             end
             @mutations_from_database = nil
             @mutations_before_last_save = nil
-            pk = self.class.primary_key
-            if pk && @attributes.fetch_value(pk) != restore_state[:id]
-              @attributes.write_from_user(pk, restore_state[:id])
+            if @attributes.fetch_value(@primary_key) != restore_state[:id]
+              @attributes.write_from_user(@primary_key, restore_state[:id])
             end
             freeze if restore_state[:frozen?]
           end