Mix :select with :include in find method (Rails 2) - ruby-on-rails

Question

I have 2 models, User and UserProfile. A user has_one user_profile and a user_profile belongs_to user.
1) Find without select
This query in console works fine, and take only 2 SQL queries.
>> User.find(:all, :limit => 10, :include => [ :user_profile ])
 
User Load (0.3ms) SELECT * FROM `users` LIMIT 10
UserProfile Load (0.3ms) SELECT `user_profiles`.* FROM `user_profiles`
WHERE (`user_profiles`.user_id IN (1,2,3,...))
2) Find with select on user model
I can select columns from User model, with
>> User.find(:all, :select => '`users`.id, `users`.last_name',
:limit => 10, :include => [ :user_profile ])
 
User Load (0.3ms) SELECT `users`.id, `users`.last_name FROM `users` LIMIT 10
UserProfile Load (0.2ms) SELECT `user_profiles`.* FROM `user_profiles`
WHERE (`user_profiles`.user_id IN (17510,18087,17508,17288...))
Everything works fine. Note that I must set users.id in the user selected columns, because the second query doesn't work (return NULL).
3) Find with select on user_profile model
But when I try to select columns from UserProfile model, I got only 1 query, which doesn't take care of my :select
>> User.find(:all,
:select => '`users`.id, `users`.last_name, `user_profiles`.permalink',
:limit => 10, :include => [ :user_profile ])
 
User Load Including Associations (0.6ms) SELECT `users`.`id` AS t0_r0,
`users`.`login` AS t0_r1, ....
`user_profiles`.`id` AS t1_r0,
`user_profiles`.`birth_date` AS t1_r1,
LEFT OUTER JOIN `user_profiles` ON user_profiles.user_id = users.id LIMIT 10
As you can see, the Rails query contains fiels from users and fields from user_profiles that I didn't select.
4) Join method
Codeit purpose a method with join function :
user_details = User.find(:all,
:select => '`users`.id, `users`.last_name, `user_profiles`.permalink',
:limit => 10, :joins => [ :user_profile ]
)
 
User Load (0.2ms) SELECT `users`.id, `users`.last_name, `user_profiles`.permalink
FROM `users`
INNER JOIN `user_profiles` ON user_profiles.user_id = users.id
LIMIT 10
This solution works fine with SQL queries, but doesn't make 'link' between User and User Profile. 10 new queries are needed, while the method 1 and 2 make only 2 SQL queries.
user_details.map(&:user_profile).map(&:permalink)
UserProfile Load (0.3ms) SELECT * FROM `user_profiles` WHERE (`user_profiles`.user_id = 1) LIMIT 1
UserProfile Load (0.2ms) SELECT * FROM `user_profiles` WHERE (`user_profiles`.user_id = 2) LIMIT 1
... (10 times) ...
UserProfile Load (0.3ms) SELECT * FROM `user_profiles` WHERE (`user_profiles`.user_id = 10) LIMIT 1
Is there a right syntax to have same results than the 2 first queries, but with a :select witch select only a few columns of my models ?

Answer 1

Use join:
User.find(:all,
:select => '`users`.id, `users`.last_name, `user_profiles`.permalink',
:limit => 10, :joins => [ :user_profile ])
include is used for eager loading. It is used to solve (N+1) queries problem for accessing user_profile when you have large users with user_profile. If you want to select columns of included table you need to use join. If you use columns of included table it will just ignored from select clause.
EDIT:
user_details = User.find(:all,
:select => '`users`.id, `users`.last_name, `user_profiles`.permalink',
:limit => 10, :joins => [ :user_profile ]
)
user_details.map(&:permalink)