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copied!<p>You are using the common pattern of a real number for user control of sorting. That is a fine technique, recommended by Damien Katz. To move between adjacent documents <code>A</code> and <code>B</code>, then you set your <code>sort</code> field to the average of <code>A.sort</code> and <code>B.sort</code>.</p> <p>This question has several parts.</p> <h2>What about floating point precision?</h2> <p>Javascript <code>Number</code>s are double-precision IEEE-754 floating point numbers. They have limited precision.</p> <p>Doubles have a lot of precision. If this is human-initiated activity then it will be a very long time before dragging and dropping hits the limit. But you have two choices:</p> <h3>1. Re-normalize the sort values in the background</h3> <p>Remember rewriting your line numbers in BASIC? Same thing.</p> <p>Have a cron job or some other task (<a href="http://nodejs.org" rel="noreferrer">NodeJS</a> is getting popular) to check for unacceptably close sort values and space them out. This could use sophisticated heuristics such as:</p> <ul> <li>Wait until the site is under low activity to fix the <code>sort</code>s</li> <li>Wait until a specific user to be inactive for X time before fixing his <code>sort</code>s</li> <li>Only make modifications which space out <code>sort</code> values but which never change the view result. In other words, if you have <code>0.001</code>, <code>0.002</code>, and <code>0.003</code>, move the <code>0.003</code> first to e.g. <code>0.100</code>, then change <code>0.002</code> to <code>0.005</code>. That may have a slight helpful effect in the UI but remember, replication may not copy these in the same order so the benefit is marginal, perhaps not worth the complexity.</li> </ul> <h3>2. Use a decimal data type with unlimited precision</h3> <p>Instead of <code>sort</code> storing a Javascript <code>Number</code>, it could store a string from <em>but not including</em> <code>"0.0"</code> through <code>"1.0"</code> (say, to 100 digits). Then a string sort is also a numeric sort. (You have "anchors" at 0.0 and 1.0 which are invalid for documents. To insert a document in the first position, set <code>sort</code> to the average of 0.0 and the current first document. For the last position, <code>sort</code> is the average of the last document and 1.0.)</p> <p>Next, your client (whoever calculates the <code>sort</code> value) needs arbitrary-precision real number types. Java, Ruby, Python, pretty much all the languages have them. This post even inspired me to make a quick project, <a href="http://github.com/jhs/bigdecimal.js" rel="noreferrer">BigDecimal for Javascript</a> which is the <code>BigDecimal</code> code from Google Web Toolkit (which itself came from Apache Harmony). But there are other implementations too.</p> <p>I personally like <code>BigDecimal</code>. In your case however you would have to change your code to use a string for <code>sort</code>. However the benefit is, you never have to re-normalize the <code>sort</code>s to work around the precision.</p> <h2>What about collisions from concurrent activity?</h2> <p>CouchDB is Relaxed. What will happen is what users expect. CouchDB documents mimic the real world. As Chris Anderson says, "there are no transactions in real life."</p> <p>For three elements in a UI:</p> <ul> <li>ItemA</li> <li>ItemB</li> <li>ItemC</li> </ul> <p>What if I move <code>A</code> after <code>C</code> and you move <code>B</code> after C? Clearly, the list will either be <code>C B A</code> or <code>C A B</code>. Which should it be? That depends on your application?</p> <p>Either, <strong>it doesn't matter</strong>: Great! CouchDB will order <code>A</code> and <code>B</code> arbitrarily and you will be fine. Users will infer (or see if your UI is good) that somebody else moved the other item.</p> <p>Or,<strong>B must come before A because [some reason]</strong>: Well then, your <code>sort</code> value is wrong. It should include all relevant data to decide sub-sorts. For example, you can <code>emit([120.000, doc.userLastName], doc)</code>. When users move docs to the same place, the sort becomes alphabetical.</p> <p>If you say, <strong>A cannot be moved so soon after B moved</strong> then that is also application code that must be implemented regardless of the data store. In other words, it's not a transactional thing, it is software logic. For dragging and dropping UI elements, my feeling is, it's not worth it and the "it doesn't matter" solution is best.</p>

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