[Dbix-class] Nested joins + column subsets

[Mark Blythe]

In trying what seemed like a simple operation today, I've either
discovered a bug or documentation deficiency.  For the sake of
brevity, I won't show full class definitions here, but let's say I
have 3 simple table classes: A, B, C.  They each have a single primary
key (a_id, b_id, c_id) with straightforward "belongs to" relationships
between them:

A = (belongs to) => B = (belongs to) => C

A simple nested prefetch works fine:

$schema->resultset('A')->find($a_id, {
   prefetch => { b => 'c' }
});

However, if either B or C has a big blob or text column that I don't
want to load, I may want to specify a subset of the columns.  Thus, I
tried the following:

$schema->resultset('A')->find($a_id, {
   columns => [ 'me.a_id', 'me.b_id', 'b.b_id', 'b.c_id', 'c.c_id', 'c.title' ],
   join => { b => 'c' }
});

I was careful to specify the keys on both sides of the relationships
just to be safe.  This query does *not* work.  It dies with the error:

No such relationship "c"

I tried using 'select' rather than 'columns', but the same error
occurred.  After much debugging, I discovered that the resultset's
$self->{_attrs}{as} was being built differently than with the
prefetch.  It showed the 'b' and 'c' relationships on the same level
rather than nested, so later on when it died, it was looking for a "c"
relationship in the A class, which of course there isn't.

I was able to fix the query by adding the "as" attribute and nesting
the table prefixes there:

$schema->resultset('A')->find($a_id, {
   select => [ 'me.a_id', 'me.b_id', 'b.b_id', 'b.c_id', 'c.c_id', 'c.title' ],
   as => [ 'a_id', 'b_id', 'b.b_id', 'b.c_id', 'b.c.c_id', 'b.c.title' ],
   join => { b => 'c' }
});

If this is expected and necessary, I didn't come across it in the
attribute docs under DBIx::Class::Resultset or in
DBIx::Class::Manual::Cookbook.

So is this a bug or a feature?  BTW, I'm using v.08008

Thanks,
Mark