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    <p>I have a solution that might not work depending on the number of items in your list.</p> <p>My idea was to apply a filter to transform a simple list into a hierarchical one.</p> <p><strike>Here's a <a href="http://jsfiddle.net/xSptw/1/" rel="nofollow">working jsFiddle</a>.</strike> (See the new one at the bottom, really!)</p> <p>The idea is simple, take an existing list, and rearrange it on-the-fly:</p> <pre class="lang-html prettyprint-override"><code>&lt;div ng-repeat="g in items | groupBy:'group' | orderBy:'group'"&gt; &lt;h2&gt;{{g.group}}&lt;/h2&gt; &lt;ul&gt; &lt;li ng-repeat="item in g.items | orderBy:'title'"&gt;{{item.id}}. {{item.title}}&lt;/li&gt; &lt;/ul&gt; &lt;/div&gt; </code></pre> <p>In this case, we've take the original array, and applied a <code>groupBy</code> filter on it. This filter is actually sort of complex, because it needs to make sure it doesn't modify the array more than necessary, so it has to store a <strong>deep copy</strong> of the original array to compare against. If you remove that deep copy and inspection, you will end up with <code>$digest</code> iteration errors every time.</p> <blockquote> <p><strong>Edit:</strong> See bottom for a modified version that performs shallower copies of the array</p> </blockquote> <p>Here's the filter code:</p> <pre class="lang-js prettyprint-override"><code>app.filter("groupBy", function() { var mArr = null, mGroupBy = null, mRetArr = null; return function(arr, groupBy) { if(!angular.equals(mArr, arr) || mGroupBy !== groupBy) { mArr = angular.copy(arr); mGroupBy = groupBy; mRetArr = []; var groups = {}; angular.forEach(arr, function(item) { var groupValue = item[groupBy] if(groups[groupValue]) { groups[groupValue].items.push(item); } else { groups[groupValue] = { items: [item] }; groups[groupValue][groupBy] = groupValue; mRetArr.push(groups[groupValue]); } }); } return mRetArr; }; }); </code></pre> <p>(<code>mArr</code>, <code>mGroupBy</code>, and <code>mRetArr</code> are the "memoized" values, here.)</p> <p>What's nice about this filter is that it doesn't <em>require</em> that the data come in already grouped. The jsFiddle linked above include a button to dynamically add items to the array at the end, but they still end up sorted into groups.</p> <p>Again, the real caveat is that it could be slow and/or expensive based on the number of items in your list. Hundreds of moderately-complex items should be OK, but if you have thousands, you should pre-sort them somewhere else before they get to the scope.</p> <hr> <h1>Edit: Much better version</h1> <p>At the cost of more code complexity, I modified the filter to copy only the key information instead of deeply copying the whole array. This time, I make a hashmap of the groups containing the items. The items are still referenced, but this time it's an explicit reference, so modifying an item in the tree will not force a resort, but changing an item's group or adding or removing an item will.</p> <p><strong>New fiddle: <a href="http://jsfiddle.net/hhWaX/2/" rel="nofollow">http://jsfiddle.net/hhWaX/2/</a></strong></p> <p>And here's the new filter:</p> <pre class="lang-js prettyprint-override"><code>app.filter("groupBy", function() { var mArr = null, mGroupBy = null, mRetArr = null, getMemoArr = function(arr, groupBy) { var ret = {}; angular.forEach(arr, function(item){ var groupValue = item[groupBy]; if(ret[groupValue]) { ret[groupValue].push(item); } else { ret[groupValue] = [item]; } }); return ret; }; return function(arr, groupBy) { var newMemoArr = getMemoArr(arr, groupBy); if(mGroupBy !== groupBy || !angular.equals(mArr, newMemoArr)) { mArr = newMemoArr; mGroupBy = groupBy; mRetArr = []; var groups = {}; angular.forEach(arr, function(item) { var groupValue = item[groupBy] if(groups[groupValue]) { groups[groupValue].items.push(item); } else { groups[groupValue] = { items: [item] }; groups[groupValue][groupBy] = groupValue; mRetArr.push(groups[groupValue]); } }); } return mRetArr; }; }); </code></pre> <p>It's a lot more complicated, but this one should have significantly better performance over large sets, because there is the absolute minimum of duplication of the original array.</p>
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    1. COIn theory, if you knew the `group` would never change for any of the items, you could replace the deep copy (`angular.copy(...)`) with a shallow copy that only copied over the items themselves. There may be some magical way to store the items along with their original groups, too.
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    2. COAnd, for the hell of it, here's a version that can take a function to determine the groupBy value: http://jsfiddle.net/9dpN2/1/
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    3. COI just realized I made the memoization global by accident. I think. If I am correct, then using this filter twice within the same app/module may not work. It's too late, I'll think about later!
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