Enum.md

@@ -44,6 +44,24 @@ create_table :users do |t|
 end
 ```
 
+#### Adding value migration Exception
+
+Sometimes, adding values are prevented because migrations run inside of transactions, but there are some occasions that PostgreSQL doesn't allow that, check [PostgreSQL Docs](https://www.postgresql.org/docs/9.1/sql-altertype.html#AEN63251). In those cases, just add `disable_ddl_transaction!` to the migration:
+
+```ruby
+class AddEnumValue < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    add_enum_values :status, %i(baz qux), after: :foo
+  end
+
+  def down
+    # this migration is irreversible
+  end
+end
+```
+
 ### The type Class
 
 Each enum type loaded from the database will have its own class type of value, created under the [`enum.namespace`](https://github.com/crashtech/torque-postgresql/wiki/Configuring#enum.namespace) namespace.