How to eager load associations with the current_user?

I ran into the same issue and although everyone keeps saying there's no need to do this, I found that there is, just like you. So this works for me:

# in application_controller.rb:
def current_user
  @current_user ||= super && User.includes(:saved_listings).find(@current_user.id)
end

Note that this will load the associations in all controllers. For my use case, that's exactly what I need. If you really want it only in some controllers, you'll have to tweak this some more.

This will also call User.find twice, but with query caching that shouldn't be a problem, and since it prevents a number of additional DB hits, it still is a performance gain.

I wanted to add what I think is a better solution. As noted in comments, existing solutions may hit your DB twice with the find request. Instead, we can use ActiveRecord::Associations::Preloader to leverage Rails' work around loading associations:

def current_user
  @current_user ||= super.tap do |user|
    ::ActiveRecord::Associations::Preloader.new.preload(user, :saved_listings)
  end
end

This will re-use the existing model in memory instead of joining and querying the entire table again.

Override serialize_from_session in your User model.

class User
  devise :database_authenticatable

  def self.serialize_from_session key, salt
    record = where(id: key).eager_load(:saved_listings, roles: :accounts).first
    record if record && record.authenticatable_salt == salt
  end
end

This will however, eager load on all requests.