From 79b38b8a71f7de20dc091c9e76ffd1c5d1686ea5 Mon Sep 17 00:00:00 2001
From: David Davis <daviddavis@redhat.com>
Date: Fri, 25 Jul 2014 15:58:34 -0400
Subject: [PATCH] Added rule about before_destroy for issue #7

---
 README.md | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/README.md b/README.md
index 9a351cd3..43f3eec3 100644
--- a/README.md
+++ b/README.md
@@ -506,6 +506,32 @@ programming resources.
   end
   ```
 
+* <a name="before_destroy"></a>
+  Since [Rails creates callbacks for dependent
+  associations](https://github.com/rails/rails/issues/3458), always call
+  `before_destroy` callbacks that perform validation with `prepend: true`.
+
+  ```Ruby
+  # bad (roles will be deleted automatically by mistake)
+  has_many :roles, dependent: :destroy
+
+  before_destroy :check_deletable
+
+  def :check_deletable
+    fail "Cannot delete super admin." if super_admin?
+  end
+
+  # good
+  has_many :roles, dependent: :destroy
+
+  before_destroy :check_deletable, prepend: true
+
+  def :check_deletable
+    fail "Cannot delete super admin." if super_admin?
+  end
+  ```
+
+
 ## Migrations
 
 * <a name="schema-version"></a>