From 3a178e901e559a155d088cfb0522972625eedd6c Mon Sep 17 00:00:00 2001
From: jbranchaud <jbranchaud@gmail.com>
Date: Thu, 27 Mar 2025 10:41:35 -0500
Subject: [PATCH] Add Filter ActiveModel Validation Errors as a Rails TIL

---
 README.md                                     |  3 +-
 .../filter-active-model-validation-errors.md  | 43 +++++++++++++++++++
 2 files changed, 45 insertions(+), 1 deletion(-)
 create mode 100644 rails/filter-active-model-validation-errors.md

diff --git a/README.md b/README.md
index 71fedc7..d5ee82d 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1628 TILs and counting..._
+_1629 TILs and counting..._
 
 See some of the other learning resources I work on:
 - [Ruby Operator Lookup](https://www.visualmode.dev/ruby-operators)
@@ -1035,6 +1035,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Ensure A Rake Task Cannot Write Data](rails/ensure-a-rake-task-cannot-write-data.md)
 - [Ensure Migrations Use The Latest Schema](rails/ensure-migrations-use-the-latest-schema.md)
 - [Ensure Record Saved With after_commit Callback](rails/ensure-record-saved-with-after-commit-callback.md)
+- [Filter ActiveModel Validation Errors](rails/filter-active-model-validation-errors.md)
 - [Filter ActiveStorage Blobs To Only Images](rails/filter-active-storage-blobs-to-only-images.md)
 - [Find Or Create A Record With FactoryBot](rails/find-or-create-a-record-with-factory-bot.md)
 - [Find Records With Multiple Associated Records](rails/find-records-with-multiple-associated-records.md)
diff --git a/rails/filter-active-model-validation-errors.md b/rails/filter-active-model-validation-errors.md
new file mode 100644
index 0000000..180e5d3
--- /dev/null
+++ b/rails/filter-active-model-validation-errors.md
@@ -0,0 +1,43 @@
+# Filter ActiveModel Validation Errors
+
+Now that `ActiveModel` has a custom `Errors` class (as of Rails 6.1) instead of
+a hash, we get some useful functionality. Namely, we get a [`#where`
+method](https://api.rubyonrails.org/classes/ActiveModel/Errors.html#method-i-where)
+that allows us to filter errors based on the attribute name, type of
+validation, and even properties of that validation.
+
+Here I have created a new `Book` without any attributes. All of its validations
+are going to fail and we are going to have an `ActiveModel::Errors` object
+attached to it with several errors.
+
+```ruby
+> book = Book.new
+=>
+#<Book:0x00000001110397a8
+...
+> book.valid?
+=> false
+> book.errors
+=> #<ActiveModel::Errors [#<ActiveModel::Error attribute=added_by, type=blank, options={:message=>:required, :if=>#<Proc:0x0000000110096260 /Users/jbranchaud/.asdf/installs/ruby/3.2.2/lib/ruby/gems/3.2.0/gems/activerecord-7.2.1/lib/active_record/associations/builder/belongs_to.rb:130 (lambda)>}>, #<ActiveModel::Error attribute=title, type=blank, options={}>, #<ActiveModel::Error attribute=title, type=too_short, options={:count=>3}>, #<ActiveModel::Error attribute=author, type=blank, options={}>, #<ActiveModel::Error attribute=publication_date, type=blank, options={}>]>
+```
+
+Let's say I want to check for a specific validation error. I can use `#where`
+to filter down by attribute name (e.g. `:title`). I can filter even further by
+including the validation type as well (e.g. `:too_short`).
+
+```ruby
+> book.errors.where(:title)
+=>
+[#<ActiveModel::Error attribute=title, type=blank, options={}>,
+ #<ActiveModel::Error attribute=title, type=too_short, options={:count=>3}>]
+> book.errors.where(:title, :too_short)
+=> [#<ActiveModel::Error attribute=title, type=too_short, options={:count=>3}>]
+> book.errors.where(:title, :too_short).first.message
+=> "is too short (minimum is 3 characters)"
+> book.errors.where(:title, :too_short).first.full_message
+=> "Title is too short (minimum is 3 characters)"
+```
+
+This filtering could be used as part of conditional checks for what flash
+message gets displayed to the user or even what route/view gets rendered in
+response to the error.