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copied!<p>This is a great use case for the <em>reduce</em> phase of <em>map-reduce</em>. It's not going to be as visually elegant as the multi-group function (you can't just pass in an array of keys to group on), but overall this pattern gives you more flexibility to transform your data. <a href="http://jsfiddle.net/tynman/djQnD/1/" rel="nofollow" title="EXAMPLE">EXAMPLE</a></p> <pre><code>var grouped = _.reduce( properties, function(buckets, property) { // Find the correct bucket for the property var bucket = _.findWhere(buckets, {size: property.size, category: property.category}); // Create a new bucket if needed. if (!bucket) { bucket = { size: property.size, category: property.category, items: [] }; buckets.push(bucket); } // Add the property to the correct bucket bucket.items.push(property); return buckets; }, [] // The starting buckets ); console.log(grouped) </code></pre> <p>But if you just want it in an underscore mixin, here's my stab at it:</p> <pre><code>_.mixin({ 'groupAndSort': function (items, sortList) { var grouped = _.reduce( items, function (buckets, item) { var searchCriteria = {}; _.each(sortList, function (searchProperty) { searchCriteria[searchProperty] = item[searchProperty]; }); var bucket = _.findWhere(buckets, searchCriteria); if (!bucket) { bucket = {}; _.each(sortList, function (property) { bucket[property] = item[property]; }); bucket._items = []; buckets.push(bucket); } bucket._items.push(item); return buckets; }, [] // Initial buckets ); grouped.sort(function (x, y) { for (var i in sortList) { var property = sortList[i]; if (x[property] != y[property]) return x[property] > y[property] ? 1 : -1; } return 0; }); return _.map(grouped, function (group) { var toReturn = { key: {}, value: group.__items }; _.each(sortList, function (searchProperty) { toReturn.key[searchProperty] = group[searchProperty]; }); return toReturn; }); }); </code></pre>

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