Selecting one of a has_many relation in Rails

Question

What is the preferred way of selecting a specific record out of a has_many relation for a specific model in Rails? (I'm using Rails 5.)

I have a model User and a model Picture which are related via the following code:

class User < ApplicationRecord
  has_many :pictures, as: :imageable
  # ...
end

class Picture < ApplicationRecord
  belongs_to :imageable, polymorphic: true
  # ...
end

What I want to do is to allow a User to set a profile picture from the images associated with it, so I can call @user.profile_picture on a User object and retrieve the profile picture.

Answer 1

You can add an additional one-to-one relationship.

# create by running:
# rails g migration AddPrimaryPictureToUser
class AddPrimaryPictureToUser < ActiveRecord::Migration[5.0]
  def change
    add_column :users, :primary_picture_id, :integer
    add_index :users, :primary_picture_id
    add_foreign_key :users, :pictures, column: :primary_picture_id
  end
end

---

class User < ApplicationRecord
  has_many :pictures, as: :imageable
  # This should be belongs_to and not has_one as the foreign key column is 
  # on the users table
  belongs_to :primary_picture, 
             class_name: 'Picture',
             optional: true
end


class Picture < ApplicationRecord
  belongs_to :imageable, polymorphic: true
  has_one :primary_user, 
          foreign_key: 'primary_picture_id',
          class_name: 'User'
  # ...
end

The main reason to do it this way vs for example a boolean flag on the pictures table is that having a separate relationship makes it easy to join which is important for avoiding N+1 queries which is an issue if you are listing a bunch of users together with their primary image.

@users = User.includes(:primary_picture).all