Before this commit, the field assignment would cache the value it received, even
if that value was altered during `write`. This can happen if `write` is
overridden (for example to resize images), or it could also happen if a database
procedure is altering the value.
To fix this issue, we do not cache the value that was assigned. This implies
that additional queries may be necessary to retrieve the value, but those were
already necessary most of the time, so it does not have a significant impact on
performances.
Sébastien Theys
c7fe955daa