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

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

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+# 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"]
+```