Don't share seen object cache between different join nodes in eager loading

Currently, the seen object cache is shared if join nodes have the same
target class. But it is a wrong assumption, we can't share the seen
object cache between different join nodes (e.g. `:readonly_account` and
`:accounts` have the same target class `Account`, but the instances
have the different state `readonly`).

Fixes #26805.
Closes #27737.

2 files changed, 20 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/associations/join_dependency.rb b/activerecord/lib/active_record/associations/join_dependency.rb
index 8cf1b5d25a..be56d27021 100644
--- a/activerecord/lib/active_record/associations/join_dependency.rb
+++ b/activerecord/lib/active_record/associations/join_dependency.rb
@@ -225,7 +225,7 @@ module ActiveRecord
               next
             end
 
-            model = seen[ar_parent.object_id][node.base_klass][id]
+            model = seen[ar_parent.object_id][node][id]
 
             if model
               construct(model, node, row, seen, model_cache)
@@ -237,7 +237,7 @@ module ActiveRecord
                 model.readonly!
               end
 
-              seen[ar_parent.object_id][node.base_klass][id] = model
+              seen[ar_parent.object_id][node][id] = model
               construct(model, node, row, seen, model_cache)
             end
           end
diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index 8be663e3dc..5b8d4722af 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -123,6 +123,24 @@ class EagerAssociationTest < ActiveRecord::TestCase
     assert_equal memberships(:membership_of_boring_club), member.current_membership
   end
 
+  def test_loading_associations_dont_leak_instance_state
+    assertions = ->(firm) {
+      assert_equal companies(:first_firm), firm
+
+      assert_predicate firm.association(:readonly_account), :loaded?
+      assert_predicate firm.association(:accounts), :loaded?
+
+      assert_equal accounts(:signals37), firm.readonly_account
+      assert_equal [accounts(:signals37)], firm.accounts
+
+      assert_predicate firm.readonly_account, :readonly?
+      assert firm.accounts.none?(&:readonly?)
+    }
+
+    assertions.call(Firm.preload(:readonly_account, :accounts).first)
+    assertions.call(Firm.eager_load(:readonly_account, :accounts).first)
+  end
+
   def test_with_ordering
     list = Post.all.merge!(includes: :comments, order: "posts.id DESC").to_a
     [:other_by_mary, :other_by_bob, :misc_by_mary, :misc_by_bob, :eager_other,