[Rails] What's the proper way to delete all associated children in one SQL statement, with AR 4.0 ?

Ruby on Rails Wednesday, August 7, 2013

Let's imagine these models, with ActiveRecord 4.0

class Parent < ActiveRecord::Base
has_many :items
end

class Item < ActiveRecord::Base
belongs_to :parent
end

I need to delete all items associated to one parent, without callback.

I don't need to call #destroy, but only executing the appropriate SQL statement.

First case :

> parent.items.delete_all

In Rails 4.1 delete_all on associations would not fire callbacks. It means if the :dependent option is :destroy then the associated records would be deleted without loading and invoking callbacks.

Item Load (0.3ms) SELECT "items".* FROM "items" WHERE "items"."parent_id" = ? [["parent_id", 1]]

(0.1ms) begin transaction
SQL (1.1ms) DELETE FROM "items" WHERE "items"."id" = ? [["id", 44]]
SQL (0.1ms) DELETE FROM "items" WHERE "items"."id" = ? [["id", 43]]
SQL (0.1ms) DELETE FROM "items" WHERE "items"."id" = ? [["id", 42]]
SQL (0.1ms) DELETE FROM "items" WHERE "items"."id" = ? [["id", 41]]
...

(80.0ms) commit transaction

I got one delete per item. Not perfect !

Now, let's try a workaround :

> parent.items.scoped.delete_all

DEPRECATION WARNING: Model.scoped is deprecated. Please use Model.all instead.
SQL (0.1ms) DELETE FROM "items" WHERE "items"."parent_id" = ? [["parent_id", 1]]

It works fine: only one statement.

But according to the warning, it seems to not be the proper way to do.

So, OK, let's try :

> feed.items.all.delete_all

DEPRECATION WARNING: Relation#all is deprecated. If you want to eager-load a relation, you can call #load (e.g. `Post.where(published: true).load`).

If you want to get an array of records from a relation, you can call #to_a (e.g. `Post.where(published: true).to_a`).
Item Load (0.3ms) SELECT "items".* FROM "items" WHERE "items"."feed_id" = ? [["feed_id", 1]]

NoMethodError: undefined method `delete_all' for #<Array:0x007f87ae6fee00>
from ~/.rvm/gems/ruby-2.0.0-p0@global/gems/rush-0.6.8/lib/rush/find_by.rb:16:in `method_missing'
from (irb):24
from ~/.rvm/gems/ruby-2.0.0-p0@global/gems/railties-4.0.0/lib/rails/commands/console.rb:90:in `start'
from ~/.rvm/gems/ruby-2.0.0-p0@global/gems/railties-4.0.0/lib/rails/commands/console.rb:9:in `start'
from ~/.rvm/gems/ruby-2.0.0-p0@global/gems/railties-4.0.0/lib/rails/commands.rb:64:in `<top (required)>'
from bin/rails:4:in `require'
from bin/rails:4:in `<main>'

I also tried to call #load : I got the same result as for the first case (too many DELETE statements).

I can also do that :

> Item.where(feed: feed).delete_all

SQL (0.2ms) DELETE FROM "feed_items" WHERE "feed_items"."feed_id" = 1

Works fine. No warning. But let's forget the OOP.

So.. What's the proper way ?

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[Rails] What's the proper way to delete all associated children in one SQL statement, with AR 4.0 ?

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