Query::Composer is a new gem for simplifying the creation of dynamic, complex SQL queries. Instead of trying to wrangle the query manually into a large mess of nested subqueries and complicated joins, Query::Composer provides an interface on top of Arel that lets you define each subquery individually, describe how those subqueries relate to each other by way of dependencies, and then will generate the final query for you, stitching the different pieces together into a coherent whole.
Win!