rails/rails — pull request #9630 by robertjwhitney (2013-03-09 17:45:53+00:00)
Title: Update documentation for collection<< to reflect behavior for new parent objects.

collection<< does not automatically fire update sql unless the parent object
is not a new record (and thus has a primary key).

eg:
```ruby
p = Post.new
p.comments << Comment.new
```
does not call update sql until:

```ruby
p.save
```

at which point it saves the parent object and then the collection object.