So I want to SELECT from multiple tables, so as to be able to do
“SELECT FROM foo, bar WHERE foo.id=bar.id AND bar.baz=23”.
How do I do it with AR#find?
Also, I need this for associations. has_many et. al. have :finder_sql,
which is nice, but feels like a Pyrrhic: it’s basically doing the
association in pure SQL, defeating the purpose of AR.