I'm fairly certain this 2.x feature never made it to rails 3. You can view
the original ticket and patches here:
http://web.archive.org/web/20081005125758/http://dev.rubyonrails.org/ticket/9560
The gist of it is, when you run something like this:
Article.includes(:comments, :author).where('authors.id = 1').limit(10)
You end up with two queries. The first is a "SELECT DISTINCT authors.id...",
and the next will actually load the comments and authors associations. In
Rails 2.x, AR was smart enough to only join against the tables that actually
limited the resultset (e.g anything in the where or order clauses). Rails 3
will blindly join all the tables, which kills performance when you have
several eager loaded associations.
I started working on a patch to apply_join_dependency but ran into a problem
with table aliasing. The diff is here:
https://gist.github.com/1067917
The approach is basically to scan the order and where clauses for table
names. Then scan the included associations for these table names, adding
them (and any intermediate joins) to a list, and only joining those
associations. The problem is when the arel object is built for
clean_relation, it has fewer joins than the original. AR builds a
JoinDependency object and JoinDependency#graft's all my joins to it. That
object never actually sees any of the original joins, and so the alias
tracker hands out the default table name instead of the aliased table name.
I don't know how to deal with this without hacking up more of the source
than I want to - anyone have any ideas about how to deal with this? Or maybe
a completely different approach?
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