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copied!<p>Its actually a pretty easy fix if you think about how the aggregation framework is described.</p> <p>Taken from the <a href="http://docs.mongodb.org/manual/core/aggregation/#pipelines" rel="nofollow">docs</a>:</p> <blockquote> <p>Conceptually, documents from a collection pass through an aggregation pipeline, which transforms these objects as they pass through. For those familiar with UNIX-like shells (e.g. bash,) the concept is analogous to the pipe (i.e. |) used to string text filters together.</p> </blockquote> <p>You may have read that before, but the reason to explain that again is that you can pass operations into that pipeline in just about any order - and more than once. Where as in MYSQL for example, <code>LIMIT</code> is always listed at the end of the query and applies to the result set after all other grouping functions.</p> <p>In MongoDB, the operations are run in the order you've added them to the pipeline. So order of operation matters.</p> <p>Looking at your your code above, it appears you're actually fetching everything - and depending on your IF statements, first ordering it, applying your offset and limit, then matching against that result set before projecting and grouping.</p> <p>So - long story short - it just looks like you need to reorder things:</p> <pre><code>pipelines = [] match = { 'discussion_id': { '$in': discussion_ids } } if start and end: match['created_at'] = { '$gte': start, '$lt': end } pipelines.append({ '$match': match }) if order: pipelines.append({ '$sort': { 'created_at': order } }) pipelines.append({ '$project': { 'poster_id': '$poster_id' } }) pipelines.append({ '$group': { '_id': '$poster_id', 'post_id': { '$first': '$_id' } } }) if skip: pipelines.append({ '$skip': skip }) if limit: pipelines.append({ '$limit': limit }) results = self.db.posts.aggregate(pipelines) </code></pre>

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