Introduce `ActiveRecord::Base#accessed_fields`

This method can be used to see all of the fields on a model which have
been read. This can be useful during development mode to quickly find
out which fields need to be selected. For performance critical pages, if
you are not using all of the fields of a database, an easy performance
win is only selecting the fields which you need. By calling this method
at the end of a controller action, it's easy to determine which fields
need to be selected.

While writing this, I also noticed a place for an easy performance win
internally which I had been wanting to introduce. You cannot mutate a
field which you have not read. Therefore, we can skip the calculation of
in place changes if we have never read from the field. This can
significantly speed up methods like `#changed?` if any of the fields
have an expensive mutable type (like `serialize`)

```
Calculating -------------------------------------
 #changed? with serialized column (before)
                       391.000  i/100ms
 #changed? with serialized column (after)
                         1.514k i/100ms
-------------------------------------------------
 #changed? with serialized column (before)
                          4.243k (± 3.7%) i/s -     21.505k
 #changed? with serialized column (after)
                         16.789k (± 3.2%) i/s -     84.784k
```

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index d5c141630e..45c3cb0690 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Add `ActiveRecord::Base#accessed_fields`, which can be used to quickly
+    discover which fields were read from a model when you are looking to only
+    select the data you need from the database.
+
+    *Sean Griffin*
+
 *   Introduce the `:if_exists` option for `drop_table`.
 
     Example: