strange thing about 1:1 association in rail
In my current project, two model `Video` and `Picture` is one-to-one association, because one video has only one thumbnail picture, and I put the foreign key `picture_id` in the `videos table`.

I type this command in console:

>v = Video.find(:first)

>p = Picture.find(:first)

then I found a very strange thing:
if I build the association this way:
> v.picture= p

>v.picture

>=> p
is ok

but the other direction return nil:

>p.video

>=>nil

but using
`p.video = v` to build the association 

there's no problem from both sides

Anybody can tell me why is it?

strange thing about 1:1 association in rail
===

In my current project, two model `Video` and `Picture` is one-to-one association, because one video has only one thumbnail picture, and I put the foreign key `picture_id` in the `videos table`.

I type this command in console:

>v = Video.find(:first)

>p = Picture.find(:first)

then I found a very strange thing:
if I build the association this way:
> v.picture= p

>v.picture

>=> p
is ok

but the other direction return nil:

>p.video

>=>nil

but using
`p.video = v` to build the association 

there's no problem from both sides

Anybody can tell me why is it?
