diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 4caa434fc00d..eebbd17f4355 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -768,15 +768,20 @@ module ClassMethods
# Objects will be in addition destroyed if they're associated with :dependent => :destroy,
# and deleted if they're associated with :dependent => :delete_all.
# [collection=objects]
- # Replaces the collections content by deleting and adding objects as appropriate.
+ # Replaces the collections content by deleting and adding objects as appropriate. If the :through
+ # option is true callbacks in the join models are triggered except destroy callbacks, since deletion is
+ # direct.
# [collection_singular_ids]
# Returns an array of the associated objects' ids
# [collection_singular_ids=ids]
- # Replace the collection with the objects identified by the primary keys in +ids+
+ # Replace the collection with the objects identified by the primary keys in +ids+. This
+ # method loads the models and calls collection=. See above.
# [collection.clear]
# Removes every object from the collection. This destroys the associated objects if they
# are associated with :dependent => :destroy, deletes them directly from the
# database if :dependent => :delete_all, otherwise sets their foreign keys to +NULL+.
+ # If the :through option is true no destroy callbacks are invoked on the join models.
+ # Join models are directly deleted.
# [collection.empty?]
# Returns +true+ if there are no associated objects.
# [collection.size]
@@ -869,9 +874,11 @@ module ClassMethods
# [:as]
# Specifies a polymorphic interface (See belongs_to).
# [:through]
- # Specifies a Join Model through which to perform the query. Options for :class_name and :foreign_key
+ # Specifies a join model through which to perform the query. Options for :class_name and :foreign_key
# are ignored, as the association uses the source reflection. You can only use a :through query through a belongs_to
- # has_one or has_many association on the join model.
+ # has_one or has_many association on the join model. The collection of join models can be managed via the collection
+ # API. For example, new join models are created for newly associated objects, and if some are gone their rows are deleted (directly,
+ # no destroy callbacks are triggered).
# [:source]
# Specifies the source association name used by has_many :through queries. Only use it if the name cannot be
# inferred from the association. has_many :subscribers, :through => :subscriptions will look for either :subscribers or
diff --git a/railties/guides/source/association_basics.textile b/railties/guides/source/association_basics.textile
index 335d17579d6a..c69f2ae8c9a9 100644
--- a/railties/guides/source/association_basics.textile
+++ b/railties/guides/source/association_basics.textile
@@ -137,6 +137,16 @@ end
!images/has_many_through.png(has_many :through Association Diagram)!
+The collection of join models can be managed via the API. For example, if you assign
+
+
+physician.patients = patients
+
+
+new join models are created for newly associated objects, and if some are gone their rows are deleted.
+
+WARNING: Automatic deletion of join models is direct, no destroy callbacks are triggered.
+
The +has_many :through+ association is also useful for setting up "shortcuts" through nested +has_many+ associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. You could set that up this way: