Make prefetch_associations work as expected on associations that have been particually prefetched

[kirs]

Consider the following code:

def work(product_variant_ids)
  product_variants = ProductVariant.fetch_multi(product_variant_ids, includes: { product: [] })
  safe = perform_safety_checks(product_variants)
  return if !safe
  
  # Now that it's safe, we want to access product variants
  # with their products and images and some other items.
  # For that, we'd want them to be prefetched to avoid N+1
  ProductVariant.prefetch_associations({ product: [:images], some_other_items: [] }, product_variants)
end

The following uses prefetch_associations API from IDC. It works great, apart from when the same association (in our case, :product) has already been part of prefetching.

In the snippet above, some_other_items would be loaded but product: [:images] would not be, because product is already considered being loaded.

This PR fixes that bug.

[drinkbeer]

Great find about the bug!

[drinkbeer]

Just try to understand the code and unit tests. When adding a binding.pry here, I saw the records were like:

[3] pry(#<IdentityCache::Cached::BelongsTo>)> records
=> [#<AssociatedRecord:0x000000010ac24880
  id: 1,
  name: "bob child",
  item_id: 1,
  item_two_id: nil,
  created_at: 2024-02-15 21:32:33.350638 UTC,
  updated_at: 2024-02-15 21:32:33.350638 UTC>,
 #<AssociatedRecord:0x000000010ac24740
  id: 2,
  name: "fred child",
  item_id: 3,
  item_two_id: nil,
  created_at: 2024-02-15 21:32:33.361986 UTC,
  updated_at: 2024-02-15 21:32:33.361986 UTC>]

So basically, in the tests, the bob has id 1, fred has id 3, bob_child has id 1, fred_child has id 2.

So in the test:

      assert_memcache_operations(1) do
        AssociatedRecord.prefetch_associations({ item: :item }, [@cached_bob_child, @cached_fred_child])
      end

There is only 1 memcached operation because only fred is loaded as an item of fred_child, and bob and bob_child has the same id, so only one associated object loaded from memcached?

[kirs]

Thanks for spotting. I'll rework the tests.

[kirs]

Reworked the tests 🙏