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    <p>I don't see any great way to do what you're trying to do directly. If you're willing to accept a little de-normalization, I would recommend a pre-save signal to mark messages as being at the head.</p> <pre><code>#In your model head = models.BooleanField(default=True) #As a signal plugin: def check_head(sender, **kwargs): message = kwargs['instance'] if hasattr(message,'no_check_head') and message.no_check_head: return previous_message = Message.objects.filter(time__lt=message.time).order_by('-time')[0] if message.source == previous_message.source: message.head = False next_message = Message.objects.filter(time__gt=message.time).order_by('time')[0] if message.source == next_message.source: next_message.head = False next_message.no_check_head next_message.save() </code></pre> <p>Then your query becomes magically simple:</p> <pre><code>messages = Message.objects.filter(head=True).order_by('time')[0:15] </code></pre> <p>To be quite honest...the signal listener would have to be a bit more complicated than the one I wrote. There are a host of lost synchronization/lost update problems inherent in my approach, the solutions to which will vary depending on your server (if it is single-processed, multi-threaded, then a python <code>Lock</code> object should get you by, but if it is multi-processed, then you will really need to implement locking based on files or database objects). Also, you will certainly also have to write a corresponding delete signal listener.</p> <p>Obviously this solution involves adding some database hits, but they are on edit as opposed to on view, which might be worthwhile for you. Otherwise, perhaps consider a cruder approach: grab 30 stories, loop through the in the view, knock out the ones you won't display, and if you have 15 left, display them, otherwise repeat. Definitely an awful worst-case scenario, but perhaps not terrible average case?</p> <p>If you had a server configuration that used a single process that's multi-threaded, a Lock or RLock should do the trick. Here's a possible implementation with non-reentrant lock:</p> <pre><code>import thread lock = thread.allocate_lock() def check_head(sender, **kwargs): # This check must come outside the safe zone # Otherwise, your code will screech to a hault message = kwargs['instance'] if hasattr(message,'no_check_head') and message.no_check_head: return # define safe zone lock.acquire() # see code above .... lock.release() </code></pre> <p>Again, a corresponding delete signal is critical as well.</p> <p>EDIT: Many or most server configurations (such as Apache) will prefork, meaning there are several processes going on. The above code will be useless in that case. See <a href="https://stackoverflow.com/questions/698950/what-is-the-simplest-way-to-lock-an-object-in-django/699222#699222">this page</a> for ideas on how to get started synchronizing with forked processes.</p>
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    1. USDavid Berger
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    1. COI think this is the general approach you'd have to take, though you could replace the database hits by storing last_source in memcached; there'd still be a potential race condition under heavy concurrency, but if it didn't have to be 100% perfect...
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    2. COAs you said, not the perfect option, but I still think is the best. Thanks
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    3. COAlso...I guess I've always liked using signals...but overriding the save method could do the same thing. Some stylistic purists would argue that that's "the right thing".
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