Rails: How to sort many-to-many relation

Question

I have a many-to-many relationship between a model User and Picture. These are linked by a join table called Picturization.

If I obtain a list of users of a single pict

Answer 1

You can specify the table name in the order method/clause:

picture.users.order("picturizations.created_at DESC")

Answer 2

Have an additional column something like sequence in picturization table and define sort order as default scope in your Picturization

default_scope :order => 'sequence ASC'

If you want default sort order based on modified_at then use following default scope

default_scope :order => 'modified_at DESC'

Answer 3

Well, in my case, I need to sort many-to-many relation by a column named weight in the middle-table. After hours of trying, I figured out two solutions to sort many-to-many relation.

Solution1: In Rails Way

picture.users.where(:order => "created_at")

cannot return a ActiveRecord::Relation sorted by Picturization's created_at column.

I have tried to rewrite a default_scope method in Picturization, but it does not work:

def self.default_scope
  return Picturization.all.order(weight: :desc)
end  

Instead, first, you need to get the ids of sorted Picturization:

ids = Picturization.where(user_id:user.id).order(created_at: :desc).ids

Then, you can get the sorted objects by using MySQL field functin

picture.users.order("field(picturizations.id, #{ids.join(",")})")

which generates SQL looks like this:

SELECT `users`.* 
FROM `pictures` INNER JOIN `picturizations` 
ON `pictures`.`id` = `picturizations`.`picture_id` 
WHERE `picturizations`.`user_id = 1#for instance 
ORDER BY field(picturizations.id, 9,18,6,8,7)#for instance

Solution2: In raw SQL Way

you can get the answer directly by using an order by function:

SELECT `users`.* 
FROM `pictures` INNER JOIN `picturizations` 
ON `pictures`.`id` = `picturizations`.`picture_id` 
WHERE `picturizations`.`user_id = 1

order by picturizations.created_at desc