Fix strict loading for Active Storage previews

`ActiveStorage::Preview#url` delegates to the preview image's variant,
which in turn delegates to the variant's blob.  Thus when the variant
has already been processed and strict loading is enabled, the
association chain of `preview_image_attachment` => `blob` =>
`variant_records` => `image_attachment` => `blob` must be fully
pre-loaded; otherwise, `ActiveStorage::Preview#url` will raise an
`ActiveRecord::StrictLoadingViolationError`.

Author: jonathanhefner

activestorage/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Prevent `ActiveRecord::StrictLoadingViolationError` when strict loading is
+    enabled and the variant of an Active Storage preview has already been
+    processed (for example, by calling `ActiveStorage::Preview#url`).
+
+    *Jonathan Hefner*
+
 *   Fix `preprocessed: true` option for named variants of previewable files.
 
     *Nico Wenterodt*

activestorage/app/models/active_storage/blob.rb

@@ -134,7 +134,10 @@ def signed_id_verifier # :nodoc:
 
     def scope_for_strict_loading # :nodoc:
       if strict_loading_by_default? && ActiveStorage.track_variants
-        includes(variant_records: { image_attachment: :blob }, preview_image_attachment: :blob)
+        includes(
+          variant_records: { image_attachment: :blob },
+          preview_image_attachment: { blob: { variant_records: { image_attachment: :blob } } }
+        )
       else
         all
       end

activestorage/test/controllers/representations/redirect_controller_test.rb

@@ -110,7 +110,7 @@ class ActiveStorage::Representations::RedirectControllerWithPreviewsTest < Actio
 class ActiveStorage::Representations::RedirectControllerWithPreviewsWithStrictLoadingTest < ActionDispatch::IntegrationTest
   setup do
     @blob = create_file_blob filename: "report.pdf", content_type: "application/pdf"
-    @blob.preview(resize_to_limit: [100, 100]).processed
+    @blob.preview(resize_to_limit: [100, 100]).processed.send(:variant).processed
   end
 
   test "showing existing preview record inline" do

activestorage/test/models/blob_test.rb

@@ -328,28 +328,13 @@ class ActiveStorage::BlobTest < ActiveSupport::TestCase
   end
 
   test "scope_for_strict_loading adds includes only when track_variants and strict_loading_by_default" do
-    assert_empty(
-      ActiveStorage::Blob.scope_for_strict_loading.includes_values,
-      "Expected ActiveStorage::Blob.scope_for_strict_loading have no includes"
-    )
+    assert_empty ActiveStorage::Blob.scope_for_strict_loading.includes_values
 
     with_strict_loading_by_default do
-      includes_values = ActiveStorage::Blob.scope_for_strict_loading.includes_values
-
-      assert(
-        includes_values.any? { |values| values[:variant_records] == { image_attachment: :blob } },
-        "Expected ActiveStorage::Blob.scope_for_strict_loading to have variant_records included"
-      )
-      assert(
-        includes_values.any? { |values| values[:preview_image_attachment] == :blob },
-        "Expected ActiveStorage::Blob.scope_for_strict_loading to have preview_image_attachment included"
-      )
+      assert_not_empty ActiveStorage::Blob.scope_for_strict_loading.includes_values
 
       without_variant_tracking do
-        assert_empty(
-          ActiveStorage::Blob.scope_for_strict_loading.includes_values,
-          "Expected ActiveStorage::Blob.scope_for_strict_loading have no includes"
-        )
+        assert_empty ActiveStorage::Blob.scope_for_strict_loading.includes_values
       end
     end
   end