[Dbix-class] Joins using sub-selects

[David Cantrell]

Hello gang!

I'm having trouble figuring out how to express one of my joins in
DBIx::Class.

The two tables involved and some relevant sample data are:

 > select * from serviceplan_price;
+----+----------------+-------+-------+---------------------+
| id | serviceplan_id | type  | value | effective_date_time |
+----+----------------+-------+-------+---------------------+
| 78 |             63 | new   |  0.14 | 1973-01-01 00:00:00 |
| 79 |             64 | new   |  0.73 | 1982-01-01 00:00:00 |
| 80 |             64 | new   |  3.18 | 2012-01-01 00:00:00 |
| 81 |             63 | new   |  2.99 | 2019-01-01 00:00:00 |
| 82 |             63 | renew |  ...

Note that the effective date can be in the future. This is how we
represent historical prices, and planned price rises. And that there are
two types, 'new' (the price you pay when you first buy something) and
'renew' (the price for subsequent renewals). The 'new' price might
include the cost of setting up hardware, for example, which isn't needed
on renewal.

 > select * from serviceplan;
+----+--------------------------------+
| id | name | irrelevant details ...  |
+----+--------------------------------+
| 63 | foo  | blahblah                |
| 64 | bar  | blahblah                |
+----+--------------------------------+

And I want to define a relationship so that, along with a serviceplan, I
can fetch its *current* new price or renewal price. For an added wrinkle
we want to be able to mock the current date/time in our tests, so we
can't just use NOW(), but I don't think that's the problem. In plain old
SQL it would look like this for fetching them with their current new price:

SELECT me.id, me.name, ...
        current_new_price.id, ...
    FROM serviceplan me
    JOIN serviceplan_price current_new_price ON (
         current_new_price.serviceplan_id = me.id AND
         current_new_price.type = 'new' AND
         current_new_price.effective_date_time = (
             SELECT MAX(effective_date_time)
               FROM serviceplan_price
              WHERE effective_date_time <= '$mocked'
                AND type ='new'
                AND serviceplan_id = me.id
         )
     )

In terms of a DBIx::Class relationship on my serviceplan result class
I've got this (repeated for the current_renew_price):

__PACKAGE__->belongs_to(
     current_new_price => 'MyApp::Result::ServiceplanPrice',
     sub ($args) {
         my $foreign    = $args->{foreign_alias};
         my $me         = $args->{self_alias};
         my $mocked_now = MyApp::Mocks->now(),
         return {
             "$foreign.serviceplan_id"      => { -ident => "$me.id" },
             "$foreign.type"                => 'new',
             "$foreign.effective_date_time" => { -ident => qq{
                 ( SELECT MAX(effective_date_time)
                     FROM serviceplan_price
                    WHERE effective_date_time <= '$mocked_now'
                      AND type                 = 'new'
                      AND serviceplan_id       = $me.id
                 )
             } }
         }
     }
);

I *think* that I have no choice but to write the relationship condition
as an anonymous sub, but embedding some raw SQL like that is just plain
hideous. Also I'd like to get rid of the repetition where I've said
twice that the 'type' field should be 'new' and that the serviceplan_id
should match, and the inconsistency where I refer to the foreign table
as $foreign is some places but by its true name inside the sub-select.

Any clues on how to turn that into something a bit more SQL::Abstract?

--
David Cantrell
David Cantrell
System Architect
The Hut Group<http://www.thehutgroup.com/>

Tel:
Email: david.cantrell at uk2group.com<mailto:david.cantrell at uk2group.com>

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