Nested :through associations

[Jon Leighton]

Hi all,

In October I worked on a patch for a long-requested feature:
associations that can go :through anything, including other :through
associations.

The Lighthouse ticket is here:
https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/1152

I believe this is a robust, well-tested patch. I want to get it merged,
but there has been very little response to the patch on Lighthouse. I
should probably have posted on here before now - but here I am now
anyway.

Can anyone help verify/review this patch? Any suggestions about how best
to get it accepted by the core team?

Also, can somebody with the relevant permissions changes the status from
"incomplete" to "open" please? I am actively maintaining the branch and
merging in the latest master from time to time. I will also happily deal
with any issues which arise as a result of this patch.

Thanks,
Jon Leighton

--
http://jonathanleighton.com/

[lardawge]

Jon,
I would make sure this currently merges into master and send a pull
request on github with the lighthouse ticket in the comments... Need
get more eyes on it.

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[Jon Leighton]

The patch has now been "verified" on Lighthouse. I.e. 3 people have
tested and +1ed it. I've done a pull request here: https://github.com/rails/rails/pull/121

I merged latest master over the weekend and will continue to merge as
necessary to keep it up to date.

Jon

[Jon Leighton]

Hi all,

Jose has responded on the pull request:

===========================================
Hey mate, thanks for the pull request. But could you please send this
request to Rails mailing list and get feedback from more people?

At first, I am -1 on this feature. Rails has_many :through, without
being nested, already have a few limitations that I really would like to
see fixed before adding more complexity. For example, in the case "User
has many groups through subscriptions", the following does not work:

group = user.groups.build
group.save

As it does not save the associated object subscriptions.
Calling .create() works as expected though. The lack of the ability to
support a dependent option (like :destroy, :delete) for the associated
record is also another limitation (currently we have dangling records).
We can find others examples in LH.
===========================================

I wanted to respond here to prevent the discussion being split over
multiple places:

I see the concern. Though we should be clear that nested through
associations will only ever be read-only, as writing to them would not
make any sense (not enough information to determine correct join
records).

What I would propose is that I do an audit of tickets on LH that are
related to has many through. I suspect some of them will be solved by my
patch (as it rewrites lots of HMT code). I could identify others which
are still an issue. Then maybe we could identify which other tickets
should be fixed before this nested patch goes in. I'm happy to look at
other tickets as a stepping stone to getting this patch in.

However it would be good if people could be up front and say now if
there are general concerns with this patch in principle, or whether
concerns are just around not introducing more bugs in HMT given the
number of existing bugs.

Hope that makes sense.

Cheers,
Jon

--
http://jonathanleighton.com/

[Jason King]

I'm very much in favor of hmt working to an infinite depth. From my own experience as well as responding to others, I know it's something that people just expect to work.

I'm also concerned about bugs, but I like your suggestion Jon.

[Lunks]

+1 for sorting bugs out and commiting this. HMT just helps doing some associations where it would be hard to do. Even if it is not fool proof, allowing us to associate models further where needed is welcomed. I have been in a couple of projects where coding would be easier with this patch.

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[Jack Kinsella]

+1 for the usefulness of Jon's patch. During a previous project I was
very disappointed to find that Rails didn't support nested :through
associations. Besides my own experiences, a quick search on Google for
"nested has many through associations" shows a litany of blog posts
and discussions on hacks to simulate the functionality.

For example:
http://justinfrench.com/notebook/nested-hasmany-through-associations-in-rails
http://geoff.evason.name/2010/04/23/nested-has_many-through-in-rails-or-how-to-do-a-3-table-join/
http://kconrails.com/2010/01/28/nesting-has_many-through-relationships-in-ruby-on-rails/

I agree too with José Valim's comments that has_many :through needs to
be fixed. The surprising inconsistencies between has_many :through and
has_many adds cognitive load to developers and, for me at least, the
lack of dependent options has pushed me into using has_many over
has_many :through quite a bit. If Jon's code and proposed audit can
help iron out these issues then all the better!

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