add validate section

Author: Ridhwana

guides/source/active_record_validations.md

@@ -932,6 +932,27 @@ See [`validates_uniqueness_of`][] for more information.
 [the PostgreSQL manual]:
     https://www.postgresql.org/docs/current/static/ddl-constraints.html
 
+### `validate`
+
+The `validate` validator is used with `belongs_to` and `has_one` associations to
+automatically validate the associated object whenever the parent object is
+saved. This option ensures that any new or updated associated objects are
+validated before the parent object can be successfully saved. By default, this
+option is set to `false`, meaning associated objects will not be automatically
+validated.
+
+```ruby
+class Post < ApplicationRecord
+  belongs_to :author, validate: true
+end
+```
+
+In this example, every time a `Post` is saved, the associated `Author` will also
+be validated. If the `Author` is invalid, the `Post` will not be saved.
+
+If you need to ensure that associated objects are validated every time the
+parent object is updated, consider using `validates_associated`.
+
 ### `validates_associated`
 
 You should use this validator when your model has associations that always need to