Merge pull request rails#49907 from p8/guides/association-callbacks

Move the association callbacks section to the callbacks guides [ci-skip]

Author: p8

guides/source/active_record_callbacks.md

@@ -374,6 +374,69 @@ Article destroyed
 => #<User id: 1>
 ```
 
+Association Callbacks
+---------------------
+
+Association callbacks are similar to normal callbacks, but they are triggered by events in the life cycle of a collection. There are four available association callbacks:
+
+* `before_add`
+* `after_add`
+* `before_remove`
+* `after_remove`
+
+You define association callbacks by adding options to the association declaration. For example:
+
+```ruby
+class Author < ApplicationRecord
+  has_many :books, before_add: :check_credit_limit
+
+  def check_credit_limit(book)
+    # ...
+  end
+end
+```
+
+Rails passes the object being added or removed to the callback.
+
+You can stack callbacks on a single event by passing them as an array:
+
+```ruby
+class Author < ApplicationRecord
+  has_many :books,
+    before_add: [:check_credit_limit, :calculate_shipping_charges]
+
+  def check_credit_limit(book)
+    # ...
+  end
+
+  def calculate_shipping_charges(book)
+    # ...
+  end
+end
+```
+
+If a `before_add` callback throws `:abort`, the object does not get added to
+the collection. Similarly, if a `before_remove` callback throws `:abort`, the
+object does not get removed from the collection:
+
+```ruby
+# book won't be added if the limit has been reached
+def check_credit_limit(book)
+  throw(:abort) if limit_reached?
+end
+```
+
+NOTE: These callbacks are called only when the associated objects are added or removed through the association collection:
+
+```ruby
+# Triggers `before_add` callback
+author.books << book
+author.books = [book, book2]
+
+# Does not trigger the `before_add` callback
+book.update(author_id: 1)
+```
+
 Conditional Callbacks
 ---------------------
 

guides/source/association_basics.md

@@ -2756,46 +2756,7 @@ class Author < ApplicationRecord
 end
 ```
 
-Rails passes the object being added or removed to the callback.
-
-You can stack callbacks on a single event by passing them as an array:
-
-```ruby
-class Author < ApplicationRecord
-  has_many :books,
-    before_add: [:check_credit_limit, :calculate_shipping_charges]
-
-  def check_credit_limit(book)
-    # ...
-  end
-
-  def calculate_shipping_charges(book)
-    # ...
-  end
-end
-```
-
-If a `before_add` callback throws `:abort`, the object does not get added to
-the collection. Similarly, if a `before_remove` callback throws `:abort`, the
-object does not get removed from the collection:
-
-```ruby
-# book won't be added if the limit has been reached
-def check_credit_limit(book)
-  throw(:abort) if limit_reached?
-end
-```
-
-NOTE: These callbacks are called only when the associated objects are added or removed through the association collection:
-
-```ruby
-# Triggers `before_add` callback
-author.books << book
-author.books = [book, book2]
-
-# Does not trigger the `before_add` callback
-book.update(author_id: 1)
-```
+Read more about association callbacks in the [Active Record Callbacks Guide](active_record_callbacks.html#association-callbacks)
 
 ### Association Extensions