Rails - Sep 2011 - includes from a table that does not have an id field

I have the following tables

create table files(id int ai pk, name varchar(255)
create table admins (file_id int, user_id int)
create table users (id int ai pk, name varchar(16),email varchar(255))

I want to get all the admin user names and emails of a given file. Say
for file 1

Admin.where(:file_id=>1).includes(:user). it works very good. Now I
want to sort the admins on the usernames

Admin.where(:file_id=>1).include(:user).order(''users.name'').
this
shows an sql query with an empty column and so bails out. here''s the
query I got on the console

SELECT `admins`.`` AS t0_r0, `admins`.`file_id` AS t0_r1,
`admins`.`user_id` AS t0_r2, `users`.`id` AS t1_r0, `users`.`name` AS
t1_r1 FROM `admins` LEFT OUTER JOIN `users` ON `users`.`id` `admins`.`user_id`
WHERE `admins`.`file_id` = 1 ORDER BY users.name

The first column name is being taken as empty (immediately after the
select).

Is this a bug? I don''t want to have id field in the admins table as
that''s not useful. How do I write a AR query to sort out this issue

thanks
Kiran

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On Sep 29, 2011, at 8:56 PM, maskiran wrote:
> I have the following tables
>
> create table files(id int ai pk, name varchar(255)
> create table admins (file_id int, user_id int)
> create table users (id int ai pk, name varchar(16),email varchar(255))
And presumably classes like:

class File
   has_many :admins
   has_many :users, :through => :admins
end
class Admin
   belongs_to :file
   belongs_to :user
end
class User
end
>
> I want to get all the admin user names and emails of a given file. Say
> for file 1
File.find(1).users

or

File.where(:id => 1).users
>
> Admin.where(:file_id=>1).includes(:user). it works very good. Now I
> want to sort the admins on the usernames
Then add and .order(''users.name'') to the query.

Alternatively, you could always get the users in that order:

class File
   has_many :admins
   has_many :users, :through => :admins, :order =>
''users.name''
end
>
>
Admin.where(:file_id=>1).include(:user).order(''users.name'').
this
> shows an sql query with an empty column and so bails out. here''s
the
> query I got on the console
>
> SELECT `admins`.`` AS t0_r0, `admins`.`file_id` AS t0_r1,
> `admins`.`user_id` AS t0_r2, `users`.`id` AS t1_r0, `users`.`name` AS
> t1_r1 FROM `admins` LEFT OUTER JOIN `users` ON `users`.`id` >
`admins`.`user_id` WHERE `admins`.`file_id` = 1 ORDER BY users.name
>
> The first column name is being taken as empty (immediately after the
> select).
>
> Is this a bug? I don''t want to have id field in the admins table
as
> that''s not useful. How do I write a AR query to sort out this
issue
>
> thanks
> Kiran

You should not need an `id` on the admins table, but you almost  
certainly want to have an index on each of the `file_id` and `user_id`  
columns.

-Rob

Rob Biedenharn		
Rob-xa9cJyRlE0mWcWVYNo9pwxS2lgjeYSpx@public.gmane.org
http://AgileConsultingLLC.com/
rab-/VpnD74mH8+00s0LW7PaslaTQe2KTcn/@public.gmane.org	
http://GaslightSoftware.com/

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On 30 September 2011 05:49, Rob Biedenharn
<Rob-GBZH0y1GwQfnZcttdmLDtcI/UQi/AW5J@public.gmane.org>
wrote:
> class File
>  has_many :admins
>  has_many :users, :through => :admins
> end
> class Admin
>  belongs_to :file
>  belongs_to :user
> end
> class User
> end
> You should not need an `id` on the admins table, but you almost certainly
> want to have an index on each of the `file_id` and `user_id` columns.
Are you sure? I''ve not got time to test it right now, but I was under
the impression you would need an id for the admins table unless the
model described it as the join table in a habtm?

 class File
   has_and_belongs_to_many :users, :join_table => "admins"
 end

 class User
   has_and_belongs_to_many :files, :join_table => "admins"
 end

File.first.users
etc

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Rob Biedenharn wrote in post #1024309:
> You should not need an `id` on the admins table, but you almost
> certainly want to have an index on each of the `file_id` and `user_id`
> columns.
I assume you meant an unique index across both file_id and user_id. In 
essence using the two foreign keys as the primary key of the join table. 
It would not be sufficient to prevent duplicate records in the join 
table with separate indexes on each foreign key.

file_id (PK1, FK1)
user_id (PK2, FK2)

However, IMO it is still best to let ActiveRecord have its unique 
surrogate primary key. But, that does not preclude the need for a unique 
index spanning the two foreign key columns.

Which ends up looking something like this:

add_index(:admin, [:file_id, :user_id], :unique => true)

In many cases it also does matter which column is listed first in the 
index. Try to guess which of the two keys will be the most "selective"
once the database is filled with data. Will users tend to have a lot of 
files associated them them, or will files have lots of users? That 
depends on how your tables are actually going to be used. List first the 
column that will tend to produce the fewest number of rows in the 
result, for the majority of your queries.

Here are two examples that I think illustrate join tables well:

This first example is the typical style of join tables used by Rails. 
There is actually a good reason for this. In an Object Relational 
Mapping (ORM) environment it''s really not a bad idea to let the model 
classes maintain record identity. That allows ActiveRecord to use its 
own built-in mechanism for mapping object instances to database rows.

Note: In order to conform to AR default naming replace relation_id with 
id.

CREATE TABLE posts_tags (
    relation_id int UNSIGNED NOT NULL AUTO_INCREMENT,
    post_id int UNSIGNED NOT NULL,
    tag_id int UNSIGNED NOT NULL,
    PRIMARY KEY(relation_id),
    UNIQUE INDEX(post_id, tag_id)
);

Here is a more traditional table definition. This is what I choose when 
not working within an ORM environment (rarely these days):

CREATE TABLE posts_tags (
    post_id int UNSIGNED NOT NULL,
    tag_id int UNSIGNED NOT NULL,
    PRIMARY KEY(post_id, tag_id)
);

These examples were take from:
http://20bits.com/articles/10-tips-for-optimizing-mysql-queries-that-dont-suck/

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