UNTESTED - Sort Backbone Collection by up to three terms.

sortBy.js

sortBy: function (direction, primary, secondary, tertiary) {
	this.models.sort(function (a, b) {
		var aPrimary, aSecondary, aTertiary, bPrimary, bSecondary, bTertiary;
		
		// setup return values according to the sort direction
		var lessThan = (direction === 'asc') ? -1 : 1,
			greaterThan = (direction === 'asc') ? 1 : -1;

		aPrimary = a.get(primary);
		bPrimary = b.get(primary);
		
		if (secondary) {
			aSecondary = a.get(secondary);
			bSecondary = b.get(secondary);
		}
		
		if (tertiary) {
			aTertiary = a.get(tertiary);
			bTertiary = b.get(tertiary);
		}

		// sort by primary if they are different
		if (aPrimary < bPrimary)
			return lessThan;
		else if (aPrimary > bPrimary)
			return greaterThan;

		// primary are not different so do secondary sort
		if (aSecondary < bSecondary)
			return lessThan;
		else if (aSecondary > bSecondary)
			return greaterThan;
		
		// primary and secondary are not different so do tertiary sort
		if (aTertiary < bTertiary)
			return lessThan;
		else if (aTertiary > bTertiary)
			return greaterThan;
		
		return 0;
	});
}