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POTwitter-like app using MongoDB
 

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    <p>I'm making an app that uses the classic "follow" mechanism (the one used by Twitter and a lot of other apps around the web). I'm using MongoDB. My system has a difference, though: an user can follow <em>groups</em> of users. That means that if you follow a group, you'll automatically follow all the users who are members of that group. Of course users can belong to more than one group.</p> <p>This is what I came up with:</p> <ul> <li>when <strong>user A</strong> follows <strong>user B</strong>, id of user B gets added to an embedded array (called <code>following</code>) in user A's document</li> <li>for unfollowing, I remove the id of the followed user from the <code>following</code> array</li> <li><p>groups work in the same way: when <strong>user A</strong> follows <strong>group X</strong>, id of group X gets added to the <code>following</code> array. (I actually add a <code>DBRef</code> so I know if the connection is to an user or a group.) </p></li> <li><p>when I have to check if <strong>user A</strong> follows <strong>group X</strong>, I just search for the group's id in <strong>user A</strong>'s following array.</p></li> <li>when I have to check if <strong>user A</strong> follows <strong>user B</strong>, things gets a little trickier. Each user's document has an embedded array listing all the groups the user belongs to. So I use an <code>$or</code> condition to check if user A is either following user B directly or via a group. Like this: <blockquote> <p><code>db.users.find({'$or':{'following.ref.$id':$user_id,'following.ref.$ref','users'},{'following.ref.$id':{'$in':$group_ids},'following.ref.$ref':'groups'}}})</code></p> </blockquote></li> </ul> <p>This works fine, but I think I have a few issues. For example how do I show a list of followers for a particular user, including pagination? I can't use skip() and limit() on an embedded document.</p> <p>I could change the design and use an <code>userfollow</code> collection, which would do the same job of the embedded <code>following</code> document. The problem with this approach, which I tried, is that with the <code>$or</code> condition I used earlier, users following two groups containing the same user would be listed twice. To avoid this I could use group or MapReduce, which I actually did and it works, but I'd love to avoid this to keep things simpler. Maybe I just need to think out of the box. Or maybe I took the wrong approach with both tries. Anyone already had to do a similar thing and came up with a better solution? </p> <p>(This is actually a follow-up to <a href="https://stackoverflow.com/questions/3838466/making-a-twitter-like-timeline-with-mongodb">this older question</a> of mine. I decided to post a new question to explain my new situation better; I hope it's not a problem.)</p>
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    1. COMy vote is on using map to write the follower list out to a temporary collection
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      1. USCharles Hooper
    2. COI heard Map/Reduce can be slow, so I can't do it on every page load. That means follower lists wouldn't be up-to-date, so I'd prefer to avoid this solution...
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