Fixing a bug in my lateral joins queries with rails

I have some posts about using lateral joins in rails to fetch the “top N” of each record. Some months ago Ben Sheldon helped me see a performance problem on the queries that I was using. Here I try to explain the problem, the solution proposed by him, and also an introduction about a tool he published to build this kind of “top N” queries using lateral joins. The problem In his words, “The outermost condition of the association (e.g. WHERE user_id IN ($1, ...)) doesn’t get pushed down in the current query. This leads to the lateral join being applied to all records before the condition is applied.”.