Active Record methods for associations

I learned about a few useful methods that I hadn’t heard of before.

reflect_on_association

This method allows you to check if an association exists, without making a database query.

• Will return nil if no associated records are found.
• Will return an AggregateReflection object if records are found.

class Post < ActiveRecord::Base
  has_many :upvotes
end
class Upvote < ActiveRecord::Base
  belongs_to :post
end

Post.reflect_on_association(:upvotes)

extract_associated

Useful if you need to get all associated objects from an ActiveRecord::Relation.

Assuming:

class Author < ActiveRecord::Base
  has_many :books
end

class Book < ActiveRecord::Base
  belongs_to :author
end

and we want to get all the books from a list of authors…

Normally, my first thought we be to do something like this:

Author.all.includes(:books).map(&:books) 

but there is a more readable way to do this with extract_associated.

Author.all.extract_associated(:books)

which actually does something similar, so the benefit is mostly the readability.

missing

Ever need to find records with no associated records?

For example:

class City < ActiveRecord::Base
  has_many :photos
end

class Photo < ActiveRecord::Base
  belongs_to :city
end

if I want to find all cities that have no photos, previously, I thought you needed to do something like this:

City.includes(:photos).where(photos: { city_id: nil })

missing does this.

City.where.missing(:photos)

associated

New in Rails 7. Sort of the inverse of missing, but associated checks for the presence of an association.

Finding all the cities that have photos is as simple as:

City.where.associated(:photos)