CODEWITHSUNDEEP
ruby-on-railsruby
jackie
jackie

Reputation: 1

How does default scope works?

How does the default scope work?

For example, I have in my model:

  default_scope -> { includes(:translations) }

This is my model:

class MyModel
 default_scope -> { includes(:translations) }

 has_many: translations
end

I know about that default scope can do some default job, for example sort by column.

But how it works with include?

Upvotes: 0

Views: 106

Answers (1)

Ilya Konyukhov
Ilya Konyukhov

Reputation: 2791

The method includes was designed to solve N + 1 query problem. For example, if you have a model:

class MyModel < ApplicationRecord
  has_many :translations
end

Then this code

MyModel.limit(50).each do |model|
  model.translations.each do |translation|
    # ...
  end
end

generates 51 queries to your database: one query to fetch 50 records from my_models table, and 50 queries to fetch translations for each of these models. This affects the performance of the application dramatically.

To fix it the code could be rewritten as:

MyModel.includes(:translations).limit(50).each do |model|
...

With includes Rails generates only 2 SQL query. First one fetches 50 my_models records, and second query fetches translations for those my_models.

Having default_scope -> { includes(:translations) } just let you skip writing includes(:translations) in the situations described above. Even if you write

MyModel.limit(50).each do |model|
...

Rails will do 2 queries since translations are included by default scope.

I need to add that default_scope has some non-obvious pitfalls, and it's not recommended by some developers.

Upvotes: 2

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