Add Filter ActiveModel Validation Errors as a Rails TIL

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.