Memoize user provided defaults before type casting

When a proc is given as a default value, the form builder ends up
displaying `Proc#to_s` when the default is used. That's because we
didn't handle the proc until type casting. This issue technically can
occur any time that a proc is the value before type casting, but in
reality the only place that will occur is when a proc default is
provided through the attributes API, so the best place to handle this
edge case is there.

I've opted to memoize instead of just moving the `Proc#call` up, as this
made me realize that it could potentially interact very poorly with
dirty checking.

The code here is a little redundant, but I don't want to rely on how
`value_before_type_cast` is implemented in the super class, even if it's
just an `attr_reader`.

Fixes #24249

Close #24306

1 files changed, 7 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 3cfebdc2d0..53b669dc72 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   The form builder now properly displays values when passing a proc form
+    default to the attributes API.
+
+    Fixes #24249.
+
+    *Sean Griffin*
+
 *   MySQL: strict mode respects other SQL modes rather than overwriting them.
     Setting `strict: true` adds `STRICT_ALL_TABLES` to `sql_mode`. Setting
     `strict: false` removes `STRICT_TRANS_TABLES`, `STRICT_ALL_TABLES`, and