Add Add ActiveRecord Error Not Tied To Any Attribute as a Rails til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_1131 TILs and counting..._
+_1132 TILs and counting..._
 
 ---
 
@@ -650,6 +650,7 @@ _1131 TILs and counting..._
 - [Add A Check Constraint To A Table](rails/add-a-check-constraint-to-a-table.md)
 - [Add A Foreign Key Reference To A Table](rails/add-a-foreign-key-reference-to-a-table.md)
 - [Add A Reference Column With An Index](rails/add-a-reference-column-with-an-index.md)
+- [Add ActiveRecord Error Not Tied To Any Attribute](rails/add-activerecord-error-not-tied-to-any-attribute.md)
 - [Add React With Webpacker To A New Rails App](rails/add-react-with-webpacker-to-a-new-rails-app.md)
 - [Add timestamptz Columns With The Migration DSL](rails/add-timestamptz-columns-with-the-migration-dsl.md)
 - [Access Secrets In A Rails 5.2 App](rails/access-secrets-in-a-rails-5-2-app.md)

rails/add-activerecord-error-not-tied-to-any-attribute.md

@@ -0,0 +1,61 @@
+# Add ActiveRecord Error Not Tied To Any Attribute
+
+Often the [errors on an ActiveRecord
+object](https://api.rubyonrails.org/v6.1.3.2/classes/ActiveModel/Errors.html)
+are tied to a specific attribute of that object. For instance, when this
+validation is violated
+
+```ruby
+validates :name, presence: true
+```
+
+Then the error will be tied to `:name`.
+
+With the
+[`ActiveModel::Errors#add`](https://api.rubyonrails.org/v6.1.3.2/classes/ActiveModel/Errors.html#method-i-add)
+method, we can write custom validation logic that ties an error to a specific
+attribute.
+
+```ruby
+validate :quantity_for_bulk_purchase
+
+def quantity_for_bulk_purchase
+  return if purchase_type != :bulk
+
+  if quantity < 12
+    errors.add(:quantity, "must be greater than 12 for bulk purchases")
+  end
+end
+```
+
+Errors don't have to be tied to specific attribute. They can be tied to the
+object as a whole. This can be better for validations, like the one above, that
+involve multiple attributes.
+
+```ruby
+validate :quantity_for_bulk_purchase
+
+def quantity_for_bulk_purchase
+  return if purchase_type != :bulk
+
+  if quantity < 12
+    errors.add(:base, "Quantity must be greater than 12 for bulk purchases")
+  end
+end
+```
+
+By using the `:base` symbol, we are ascribing this error to the object as a
+whole.
+
+```
+> my_object.errors
+#=> #<ActiveModel::Errors:0x00007fccaa5a8740
+ @base=
+  #<MyObject:0x00007fcc8a5e9238
+    ...
+    @details={:base=>[{:error=>"Quantity must be greater than 12 for bulk purchases"}]},
+    @messages={:base=>["Quantity must be greater than 12 for bulk purchases"]}>
+
+> my_object.errors.full_messages
+#=> ["Quantity must be greater than 12 for bulk purchases"]
+```