Add another gotcha about preloading ActiveStorage variants

[Galathius]

The way how ActiveStorage::VariantWithRecord#record is implemented breaks auto-preloading Active Storage variants:

  @record ||= if blob.variant_records.loaded? # <--- Here is the issue, association is not preloaded yet - so `find_by` is used
    blob.variant_records.find { |v| v.variation_digest == variation.digest }
  else
    blob.variant_records.find_by(variation_digest: variation.digest)
  end

To prevent n+1 you still need to use built-in Active Storage methods for preloading associations.

Examples:

User.with_attached_avatar.find(100).map { _1.avatar.variant(:small).processed.url }

or

message.images.with_all_variant_records.map do |file|
  file.representation(resize_to_limit: [100, 100]).processed.url
end

[DmitryTsepelev]

Makes sense, thank you!