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One in a `has_many` - Access Special Association Objects in ActiveRecord

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ActiveRecord models that define a has_many association often need access to a specific entry of this list. Think of an user that has many email addresses but only one that is his primary address. Or a Blog post with many comments of which one is featured.

How a lot of people do it

A pattern that seems to be quite common is to extend the association by implementing a method that gets you the specific record:

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class User < ActiveRecord::Base
  has_many :emails do
    def primary
      find(:first, conditions: 'is_primary')
    end
  end
end

This allows you to access the user’s primary email address via #emails.primary. So far so good, but what happens if we need to get a list of users with their primary email address? Of course we do eager loading to reduce the amount of database queries:

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User
  .includes(:emails)
  .find(:all)
  .each { |u| p "#{u.name}: #{u.emails.primary.email}" }

But when we look at the SQL queries that are actually executed we realize that eager loading is happening but each primary email is queried separately afterwards:

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DEBUG: User Load (0.3ms) SELECT "users".* FROM "users"
DEBUG: Email Load (0.5ms) SELECT "emails".* FROM "emails"
                          WHERE "emails"."user_id" IN (1, 2, 3, 4, 5)
DEBUG: Email Load (0.6ms) SELECT "emails".* FROM "emails"
                          WHERE "emails"."user_id" = 1 AND (is_primary)
                          LIMIT 1
DEBUG: Email Load (0.3ms) SELECT "emails".* FROM "emails"
                          WHERE "emails"."user_id" = 2 AND (is_primary)
                          LIMIT 1
DEBUG: Email Load (0.3ms) SELECT "emails".* FROM "emails"
                          WHERE "emails"."user_id" = 3 AND (is_primary)
                          LIMIT 1
DEBUG: Email Load (0.3ms) SELECT "emails".* FROM "emails"
                          WHERE "emails"."user_id" = 4 AND (is_primary)
                          LIMIT 1
DEBUG: Email Load (0.3ms) SELECT "emails".* FROM "emails"
                          WHERE "emails"."user_id" = 5 AND (is_primary)
                          LIMIT 1

Ouch! This will screw up our app’s performance as the user base grows!

A better way

But there’s another way of picking out one special instance of a has_many association. A way that also allows eager loading. It’s as simple as defining just another association pointing to the same object.

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class User < ActiveRecord::Base
  has_many :emails
  has_one :primary_email, class_name: Email, conditions: 'is_primary'
end

Now you can access the user’s primary email address by #primary_email. Let’s check the SQL log for a user list using eager loading:

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User
  .includes(:primary_email)
  .find(:all)
  .each { |u| p "#{u.name}: #{u.primary_email.email}" }

As we can see eager loading is now working properly for the primary email addresses:

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DEBUG: User Load (0.3ms) SELECT "users".* FROM "users"
DEBUG: Email Load (0.5ms) SELECT "emails".* FROM "emails"
                          WHERE "emails"."user_id" IN (1, 2, 3, 4, 5)
                          AND (is_primary)

Yay! Now all the millions of users out there can sign up on our page without breaking the list of primary email addresses…

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