Commit f727e9d9b6 made the rating field
of mail.message non-stored for performance reasons.
When the rating is activated on the website, it adds the possibility to filter
product reviews by rating (1 to 5 stars).
It does so by searching the messages that have a given rating value.
However the osv complains that you can't search on a non-stored field.
As a result, instead of displaying messages filtered by ratings,
it would always display all of them.
We add a search='_search_rating_value' on the field to allow that field to be
searched anyway.
opw 1931038
closesodoo/odoo#30499