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This is awesome. We have an implementation of this at Shopify. This is the implementation.

class AssociationPreloader
  def initialize(records)
    @records = records
  end

  def preload(*associations)
    if @records.is_a?(ActiveRecord::Relation)
      @records.preload(associations)
    else
      ActiveRecord::Associations::Preloader.new.preload(@records, associations.flatten) unless @records.empty?
      @records
    end
  end
end

Maybe we should also support passing an relation?

Also can you add documentation?

We are using similar technique in our project.
We found a major problems with this implementation:

Sometimes loaded associations.

Preloader currently determines if the association is preloaded or not by the first record https://github.com/rails/rails/blob/master/activerecord/lib/active_record/associations/preloader.rb#L180
We have encountered that practically: it results in the double load on the loaded target records. I think we need to fix that part first before exposing the power of Preloader to the audience.

Offtopic

You may see the implementation that is efficient for us:

  def preload_into_association(host_association, *preload_associations)
    # Preloading records in such a weird way
    # Because we want to cache them inside an association
    # but not have a separated scope as a result of #preload call
    preload_associations.each do |preload_association|
      collection = send(host_association)
      unpreloaded = collection.reject {|l| l.association(preload_association).loaded?}
      ActiveRecord::Associations::Preloader.new.preload(unpreloaded, preload_association)
    end
  end

Here is how it is used:

Site.has_many :campaigns
Campaign.has_many :assets

site.preload_into_association(:campaigns, :assets)
# which is better than
site.campaigns.preload(:assets) 
# because it generates new relation and site.campaigns.map(&:assets) is not cached...EVER...  

You may see that we filter out loaded records with loaded association from collection which resolves the original problem I described. The counter problem of that is that it doesn't allow to pass the nested associations... only an array of associations.

@rafaelfranca I like the Shopify implementation's straightforward way of handling relations. I'm a little concerned that the method can return different types. When passed a relation, it returns a relation so it can be chained with other query methods like where, order, etc. But when passed an Array it returns an Array.

Addressed feedback from @rafaelfranca and @kaspth. aab3b41f3d360c56bcd8e72295863676d4b3ed7a fixes the issue raised in #32140 where it is assumed that an entire list is loaded when the first record in the list is loaded. 8d6e1468b6c126f7cfb96171e417d892f257bbdf fixes preloading different object ids.

Added documentation and updated changelog.

I'm super excited about this feature, thank you so much @dinahshi and @eileencodes (and everyone else who contributed). Just wanted to confirm, is this is targeting Rails 6?

After a few months on the back burner, I believe this is ready for review! 😅

As suggested by @matthewd, I've removed the preload_associations alias to follow the precedent of "load" implying that an action occurs immediately, and renamed preload_associations (introduced by #31250) to execute_preload in order to distinguish that the operation occurs on a relation's previously defined associations.

I just realized that ActiveRecord::Base.load_associations() doesn't work at the moment. It throws ActiveRecord::Base doesn't belong in a hierarchy descending from ActiveRecord during scoping since it's currently set up in the Querying module using delegate :load_associations, to: :all.

I'm not sure how to rework this, would #load_associations have to be defined on the Core module as well as the Querying module? Is this effort worthwhile given that the method could be called on any subclass (i.e. do we want to support ActiveRecord::Base.load_associations() at all)?

cc/ @matthewd

ActiveRecord::Base doesn't belong in a hierarchy descending from ActiveRecord

Well that's an impressively-confusing error message 😅

Yes, let's move it out of Relation and into one of the model modules. At a glance it seems like a pretty equally awkward fit for both Querying and Associations, so Core looks to be the way to go. It's just a move, so there's no added complexity -- while I wouldn't encourage people to call it that way, all class methods are accessible via the relation automatically.

My thinking here is that

1. it's conceptually not really something that acts on the current relation: the only existing methods it calls on self are check_if_method_has_arguments! and build_preloader, which are fine to inline and duplicate;
2. it does seem useful for third party libraries to be able to AR::Base.load_associations(..) without needing to find a suitable subclass in the application; and
3. if we were to fix the above error, it would complain that AR::Base is an abstract class [and therefore can't have a base class]... which would mean other user-defined abstract classes become invalid too (which would be totally correct, but more annoying when documenting valid callees for this method).

👋 again! Moved load_associations to Core, addressed comments from @kaspth, and tests are passing. Can I have another round of review please? cc/ @matthewd

This PR has a lot of problems right now. Here is a brief list (some with failing tests):

• records with loaded associations may cause double loading (test 1)
• records with dirty associations will not properly merge in-memory and DB records (test 2) (see CollectionAssociation#merge_target_lists)
• through associations improperly uses loaded state which can have its own custom scope (test 3)
• through associations may override (with incomplete array because of through scope) and than reset the already loaded associations (no test invented yet)

My vision on fixing that is pretty blurry, but I can share these thoughts:
The handling of loaded and unloaded associations inside the same records array would need to improved. We can simply ensure association is not loaded before assigning association records to target inside #associate_records_to_owner. However this is ineffective due unnecessary loading of already loaded records. There should be a better way in generating separated preloader instances for records with loaded and unloaded associations.

Also, through associations loading should never assign records to source and through reflection when the special scope is applied (in terms of code - Association#reset should never be used).
It would require a serious refactoring of the preloader classes to ensure the code for loading records and assigning them as association targets is separated and can be used independently.

Failing test cases (pluggable to relations_test.rb):

  def test_load_associations_ignore_loaded_records
    post1 = posts(:welcome)
    post2 = posts(:thinking)
    posts = [post1, post2]
    comment_ids = post2.comments.to_a.pluck(:id)
    Post.load_associations(posts, :comments)
    assert_equal comment_ids, post2.comments.to_a.pluck(:id)
  end

  def test_load_associations_into_dirty_association_merges_targets_properly
    post = posts(:welcome)
    ids = post.comment_ids
    post.comments << post.comments.first
    Post.load_associations(posts, :comments)
    assert_equal ids, post.comments.ids
  end

  require "models/department"
  require "models/hotel"
  require "models/chef"
  require "models/cake_designer"
  require "models/drink_designer"
  def test_load_associations_doesnt_reset_loaded_through_associations
    cake_designer = CakeDesigner.create!(chef: Chef.new)
    drink_designer = DrinkDesigner.create!(chef: Chef.new)
    chefs = [cake_designer.chef, drink_designer.chef]
    department = Department.create!(chefs: chefs)
    hotel = Hotel.create!(departments: [department])
    Hotel.load_associations([hotel], :cake_designers, :drink_designers)
    assert_equal chefs, hotel.chefs
    assert_equal [cake_designer], hotel.cake_designers
    assert_equal [drink_designer], hotel.drink_designers
  end

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
Thank you for your contributions.

Closing because the work is going to be done in effort to make preload public and we have a lot of conflicts.