Ruby on Rails - Increment by 1 from last object - ruby-on-rails

Question

I am building a user gallery application, where user can create albums and upload images to their album. I am using nested forms for images.
In my images database, I have a column called sort_order. I want to use sort_order to manage the sorting and showing user, for ex: he is seeing 3 of 12 images, where 3 is number of sort_order and 12 is number of total images in that album.
sort_order is integer and default: 0. What I looking for is to increase/increment number of sort_order for each image based on previous sort_order of last image in same album.
For ex: image 1 will have sort_order: 1, image 2 will then have previous images sort_id +1 => sort_order: 2 and so on.
Here is the code I am using now:
after_create :previous_image
def previous_image
img = user.images.order("id ASC")
img.each do |image|
image.increment!(:sort_order, self.sort_order + 1)
end
end
This does the work but in wrong sort.
id sort_order
10 3
11 2
12 1
instead of having, id: 10 sort_order: 1, its getting 3.
This is the log:
SQL (0.5ms) UPDATE "images" SET "sort_order" = ?, "updated_at" = ? WHERE "images"."id" = ? [["sort_order", 1], ["id", 398]]
user Load (0.1ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 160]]
Slide Load (0.1ms) SELECT "images".* FROM "images" WHERE "images"."user_id" = ? ORDER BY id DESC [["user_id", 160]]
SQL (0.1ms) UPDATE "images" SET "sort_order" = ?, "updated_at" = ? WHERE "images"."id" = ? [["sort_order", 1], ["id", 399]]
SQL (0.1ms) UPDATE "images" SET "sort_order" = ?, "updated_at" = ? WHERE "images"."id" = ? [["sort_order", 2], ["id", 398]]
user Load (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = ? LIMIT 1 [["id", 160]]
Slide Load (0.1ms) SELECT "images".* FROM "images" WHERE "images"."user_id" = ? ORDER BY id DESC [["user_id", 160]]
SQL (0.1ms) UPDATE "images" SET "sort_order" = ?, "updated_at" = ? WHERE "images"."id" = ? [["sort_order", 1], ["id", 400]]
SQL (0.1ms) UPDATE "images" SET "sort_order" = ?, "updated_at" = ? WHERE "images"."id" = ? [["sort_order", 2], ["id", 399]]
SQL (0.1ms) UPDATE "images" SET "sort_order" = ?, "updated_at" = ? WHERE "images"."id" = ? [["sort_order", 3], ["id", 398]]
It starts from bottom and not top. How can I fix it?
If there is another way to do it without having the sort_order column, I am open to it.

Answer 1

After receiving the form data, add an index (starting from 1) to the images array.
Considering this is your images array from the form:
images = [{name: 'Image 1', data: 123}, {name: 'Image 2', data: 456}]
Use with_index(1) to attach the sort order to every element:
images.map.with_index(1).to_a
=> [[{:name=>"Image 1", :data=>123}, 1], [{:name=>"Image 2", :data=>456}, 2]]