IncludeByDefault, as mentioned in my last post, hit some snags with ActiveRecord generating duplicate table aliases when doing cascaded includes, e.g.
Tag.find(8).posts.find(:all, :include => :tags)
So, I set out to work around it, only to run into further problems. I went with
option C: let
find operations get all the way to the database, and then catch
StatementInvalid if it is thrown and convert problematic
:joins fragments by hand.
But, turns out Rails won’t let you use the
:joins option on
finds scoped on
HABTM associations (like the one above), meaning I had to hack support for
that in order to get my workaround for the duplicate naming bug to work. Even
then, eager loading refuses to work properly – asking for the
tags of any of
posts will go and fetch them from the database, even though the
find operation included
JOIN fragments to load the
The other complication is that, if you have duplicate many-to-many links in your
database, Rails returns incorrect result sets if you use
:include, as long as the
:include contains no
has_and_belongs_to_many associations. Which means that converting
:includes (those that raise
StatementInvalid in the example up
top, and are to-many associations) to
:joins triggers this bug. I had to hack
in a way for ActiveRecord to remember to select unique records properly, but
this still only works as long as you
:include suitable associations to begin
I have been round and round in circles intercepting various bits of the ActiveRecord call stack trying to get rid of this, but it’s whack-a-mole: fixing the eager loading thing sets off the unique records problem, or makes the call stack retry the database calls too many times, or drops the association on the floor so you can’t even read it never mind eager load it. I’ve got the plugin to a state where:
- If your
:includedoesn’t cause any SQL problems, it runs just fine.
- If it raises
StatementInvalid, we’ll rewrite it. The association causing the problem probably won’t get eager loaded, but your query will at least run.
- You will avoid the duplicate links problem as long as your
:includeincludes a to-many association.
All these apply whether the
:include is added using
explicitly in your
find call – the plugin will catch the exceptions either
way. If anyone knows a way to get all this to work without actually including
large parts of rewritten Rails (so far there’s one actual method overwrite –
the rest are wrappers that catch exceptions) then I’d love to hear about it.