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
```

3 files changed, 9 insertions, 13 deletions

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