2 files changed, 31 insertions, 3 deletions
diff --git a/activerecord/lib/active_record/counter_cache.rb b/activerecord/lib/active_record/counter_cache.rb
index 237cd56683..ed0d4aef7f 100644
--- a/activerecord/lib/active_record/counter_cache.rb
+++ b/activerecord/lib/active_record/counter_cache.rb
@@ -56,15 +56,15 @@ module ActiveRecord
     #   Post.update_counters 5, :comment_count => -1, :action_count => 1
     #   # Executes the following SQL:
     #   # UPDATE posts
-    #   #    SET comment_count = comment_count - 1,
-    #   #        action_count = action_count + 1
+    #   #    SET comment_count = COALESCE(comment_count, 0) - 1,
+    #   #        action_count = COALESCE(action_count, 0) + 1
     #   #  WHERE id = 5
     #
     #   # For the Posts with id of 10 and 15, increment the comment_count by 1
     #   Post.update_counters [10, 15], :comment_count => 1
     #   # Executes the following SQL:
     #   # UPDATE posts
-    #   #    SET comment_count = comment_count + 1,
+    #   #    SET comment_count = COALESCE(comment_count, 0) + 1,
     #   #  WHERE id IN (10, 15)
     def update_counters(id, counters)
       updates = counters.map do |counter_name, value|
diff --git a/activerecord/lib/active_record/nested_attributes.rb b/activerecord/lib/active_record/nested_attributes.rb
index 520969adbb..33611b410c 100644
--- a/activerecord/lib/active_record/nested_attributes.rb
+++ b/activerecord/lib/active_record/nested_attributes.rb
@@ -190,6 +190,34 @@ module ActiveRecord
     # destruction, are saved and destroyed automatically and atomically when
     # the parent model is saved. This happens inside the transaction initiated
     # by the parents save method. See ActiveRecord::AutosaveAssociation.
+    #
+    # === Using with attr_accessible
+    #
+    # The use of <tt>attr_accessible</tt> can interfere with nested attributes
+    # if you're not careful. For example, if the <tt>Member</tt> model above
+    # was using <tt>attr_accessible</tt> like this:
+    #
+    #   attr_accessible :name
+    #
+    # You would need to modify it to look like this:
+    #
+    #   attr_accessible :name, :posts_attributes
+    #
+    # === Validating the presence of a parent model
+    #
+    # If you want to validate that a child record is associated with a parent
+    # record, you can use <tt>validates_presence_of</tt> and
+    # <tt>inverse_of</tt> as this example illustrates:
+    #
+    #   class Member < ActiveRecord::Base
+    #     has_many :posts, :inverse_of => :member
+    #     accepts_nested_attributes_for :posts
+    #   end
+    #
+    #   class Post < ActiveRecord::Base
+    #     belongs_to :member, :inverse_of => :posts
+    #     validates_presence_of :member
+    #   end
     module ClassMethods
       REJECT_ALL_BLANK_PROC = proc { |attributes| attributes.all? { |_, value| value.blank? } }