Django: How can I protect against concurrent modification of database entries

This is how I do optimistic locking in Django:

updated = Entry.objects.filter(Q(id=e.id) && Q(version=e.version))\
          .update(updated_field=new_value, version=e.version+1)
if not updated:
    raise ConcurrentModificationException()

The code listed above can be implemented as a method in Custom Manager.

I am making the following assumptions:

• filter().update() will result in a single database query because filter is lazy
• a database query is atomic

These assumptions are enough to ensure that no one else has updated the entry before. If multiple rows are updated this way you should use transactions.

WARNING Django Doc:

Be aware that the update() method is converted directly to an SQL statement. It is a bulk operation for direct updates. It doesn't run any save() methods on your models, or emit the pre_save or post_save signals

Actually, transactions don't help you much here ... unless you want to have transactions running over multiple HTTP requests (which you most probably don't want).

What we usually use in those cases is "Optimistic Locking". The Django ORM doesn't support that as far as I know. But there has been some discussion about adding this feature.

So you are on your own. Basically, what you should do is add a "version" field to your model and pass it to the user as a hidden field. The normal cycle for an update is :

1. read the data and show it to the user
2. user modify data
3. user post the data
4. the app saves it back in the database.

To implement optimistic locking, when you save the data, you check if the version that you got back from the user is the same as the one in the database, and then update the database and increment the version. If they are not, it means that there has been a change since the data was loaded.

You can do that with a single SQL call with something like :

UPDATE ... WHERE version = 'version_from_user';

This call will update the database only if the version is still the same.

Another thing to look for is the word "atomic". An atomic operation means that your database change will either happen successfully, or fail obviously. A quick search shows this question asking about atomic operations in Django.

To be safe the database needs to support transactions.

If the fields is "free-form" e.g. text etc. and you need to allow several users to be able to edit the same fields (you can't have single user ownership to the data), you could store the original data in a variable. When the user committs, check if the input data has changed from the original data (if not, you don't need to bother the DB by rewriting old data), if the original data compared to the current data in the db is the same you can save, if it has changed you can show the user the difference and ask the user what to do.

If the fields is numbers e.g. account balance, number of items in a store etc., you can handle it more automatically if you calculate the difference between the original value (stored when the user started filling out the form) and the new value you can start a transaction read the current value and add the difference, then end transaction. If you can't have negative values, you should abort the transaction if the result is negative, and tell the user.

I don't know django, so I can't give you teh cod3s.. ;)