Sequel -- can I alias subqueries in a join? - sql

Question

Using Sequel I'd like to join two subqueries together that share some column names, and then table-qualify those columns in the select.
I understand how to do this if the two datasets are just tables. E.g. if I have a users table and an items table, with items belonging to users, and I want to list the items' names and their owners' names:
#db[:items].join(:users, :id => :user_id).
select{[items__name, users__name.as(user_name)]}
produces
SELECT "items"."name", "users"."name" AS "user_name"
FROM "items"
INNER JOIN "users" ON ("users"."id" = "items"."user_id")
as desired.
However, I'm unsure how to do this if I'm joining two arbitrary datasets representing subqueries (call them my_items and my_users)
The syntax would presumably take the form
my_items.join(my_users, :id => :user_id).
select{[ ... , ... ]}
where I would supply qualified column names to access my_users.name and my_items.name. What's the appropriate syntax to do this?
A partial solution is to use t1__name for the first argument, as it seems that the dataset supplied to a join is aliased with t1, t2, etc. But that doesn't help me qualify the item name, which I need to supply to the second argument.
I think the most desirable solution would enable me to provide aliases for the datasets in a join, e.g. like the following (though of course this doesn't work for a number of reasons)
my_items.as(alias1).join(my_users.as(alias2), :id => :user_id).
select{[alias1__name, alias2__name ]}
Is there any way to do this?
Thanks!
Update
I think from_self gets me part of the way there, e.g.
my_items.from_self(:alias => :alias1).join(my_users, :id => :user_id).
select{[alias1__name, t1__name]}
seems to do the right thing.

Answer 1

OK, thanks to Ronald Holshausen's hint, got it. The key is to use .from_self on the first dataset, and provide the :table_alias option in the join:
my_items.from_self(:alias => :alias1).
join(my_users, {:id => :user_id}, :table_alias => :alias2).
select(:alias1__name, :alias2__name)
yields the SQL
SELECT "alias1"."name", "alias2"."name"
FROM ( <my_items dataset> ) AS "alias1"
INNER JOIN ( <my_users dataset> ) AS "alias2"
ON ("alias2"."id" = "alias1"."user_id")
Note that the join hash (the second argument of join) needs explicit curly braces to distinguish it from the option hash that includes :table_alias.

Answer 2

The only way I found was to use the from method on the DB, and the :table_alias on the join method, but these don't work with models so I had to use the table_name from the model class. I.e.,
1.9.3p125 :018 > #db.from(Dw::Models::Contract.table_name => 'C1')
=> #<Sequel::SQLite::Dataset: "SELECT * FROM `vDimContract` AS 'C1'">
1.9.3p125 :019 > #db.from(Dw::Models::Contract.table_name => 'C1').join(Dw::Models::Contract.table_name, {:c1__id => :c2__id}, :table_alias => 'C2')
=> #<Sequel::SQLite::Dataset: "SELECT * FROM `vDimContract` AS 'C1' INNER JOIN `vDimContract` AS 'C2' ON (`C1`.`Id` = `C2`.`Id`)">
1.9.3p125 :020 > #db.from(Dw::Models::Contract.table_name => 'C1').join(Dw::Models::Product.table_name, {:product_id => :c1__product_id}, :table_alias => 'P1')
=> #<Sequel::SQLite::Dataset: "SELECT * FROM `vDimContract` AS 'C1' INNER JOIN `vDimProduct` AS 'P1' ON (`P1`.`ProductId` = `C1`.`ProductId`)">
The only thing I don't like about from_self is it uses a subquery:
1.9.3p125 :021 > Dw::Models::Contract.from_self(:alias => 'C1')
=> #<Sequel::SQLite::Dataset: "SELECT * FROM (SELECT * FROM `vDimContract`) AS 'C1'">