Control access to parts of a system, but also to certain pieces of information
It's hard to be specific without knowing more about your setup and about why exactly you need different users to have different permissions on different rows. But generally, I would say that whenever you access any data in the database in your code, you should precede it by an authorization check, which examines the current user and group and the row being inserted/updated/deleted/etc. and decides whether the operation should be allowed or not. Consider designing your system in an encapsulated manner - for example you could put all the functions that directly access the database in one module, and make sure that each of them contains the proper authorization check. (Having them all in one file makes it less likely that you'll miss one)
It might be helpful to add a permission_class column to the table, and have another table specifying which users or groups have which permission classes. Then your authorization check simply has to take the value of the permission class for the current row, and see if the permissions table contains an association between that permission class and either the current user or any of his/her groups.
This is a tricky question, we've been talking about this for a while (days) and haven't found a convincingly good solution. This is the situation:

We have users and groups. A user can belong to many groups (many to many relation)
There are certain parts of the site that need access control, but:
There are certain ROWS of certain tables that need access control, ie. a certain user (or certain group) should not be able to delete a certain row, but other rows of the same table could have a different permission setting for that user (or group)

Is there an easy way to acomplish this? Are we missing something?
We need to implement this in python (if that's any help).