Pinning to upgrade only unstable packages

[Spelic <spelic@shiftmail.org>]

Hello there

thanks for the really great APT .

I have one problem which I cannot solve by reading manuals and browsing 
the internet.

I have a mixed system, it's an Ubuntu oneiric - precise
(I hope my APT is like Debian's and I can ask here for help?)

I think I understood how to use pinning so to perform what is explained 
here http://www.debian.org/doc/manuals/debian-reference/ch02.en.html   
--> mixed system so that unstable packages are upgraded with unstable 
packages, stable packages are upgraded with stable packages.

Now I want to set up a **temporary**!! pinning file so that I can do an 
"apt-get upgrade" to upgrade only unstable packages while leaving all 
stables at the current version EVEN if there is a stable update for them.

Note that ubuntu has an <releasename>-updates and an 
<releasename>-security repository which frequently update packages for 
the "stable" ones, and these brings in HUNDREDS of packages each time, 
and such a mega-installation is exactly what I am trying to avoid in 
this case.

I supposed this preferences file would do the trick:
--------------------------------------------
Package: *
Pin: release a=oneiric
Pin: origin ""
Pin-Priority: 1001

Package: *
Pin: release a=oneiric-updates
Pin: origin ""
Pin-Priority: 1001

Package: *
Pin: release a=oneiric-security
Pin: origin ""
Pin-Priority: 1001
--------------------------------------------
but it doesn't work: "apt-get upgrade --simulate" show that all packages 
are being upgraded to precise (unstable).
It seems like the origin line does not match. This is contrary to 
manpage and I dotn't know how to make it match.

By reading through the code I found this other possibility:

Pin: release a=now

which matches locally installed packages (same that I thought Pin: 
origin "" would do) but after having bound the "a" to "now" I can't also 
bind it to something else, so I cannot distinguish anymore an oneiric* 
package (which I don't want to upgrade) from a precise* package (which I 
do want to upgrade).

I tried combinations such as one or more of:

Package: *
Pin: release a=now,n=oneiric
Pin-Priority: 1000

Package: *
Pin: release a=now,n=oneiric-updates
Pin-Priority: 1000

but without success.
Can you tell me how I should do?

Thank you
S.