Ensure that `ActionController::Parameters` can still be passed to AR

Since nested hashes are also instances of
`ActionController::Parameters`, and we're explicitly looking to work
with a hash for nested attributes, this caused breakage in several
points.

This is the minimum viable fix for the issue (and one that I'm not
terribly fond of). I can't think of a better place to handle this at the
moment. I'd prefer to use some sort of solution that doesn't special
case AC::Parameters, but we can't use something like `to_h` or `to_a`
since `Enumerable` adds both.

While I've added a trivial test case for verifying this fix in
isolation, we really need better integration coverage to prevent
regressions like this in the future. We don't actually have a lot of
great places for integration coverage at the moment, so I'm deferring it
for now.

Fixes #20922.

3 files changed, 33 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 237b07f653..47364325ea 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Ensure that `ActionController::Parameters` can still be passed to nested
+    attributes.
+
+    Fixes #20922.
+
+    *Sean Griffin*
+
 *   Deprecate force association reload by passing a truthy argument to
     association method.
 
diff --git a/activerecord/lib/active_record/nested_attributes.rb b/activerecord/lib/active_record/nested_attributes.rb
index c942d0e265..c337e1d18f 100644
--- a/activerecord/lib/active_record/nested_attributes.rb
+++ b/activerecord/lib/active_record/nested_attributes.rb
@@ -386,6 +386,9 @@ module ActiveRecord
     # then the existing record will be marked for destruction.
     def assign_nested_attributes_for_one_to_one_association(association_name, attributes)
       options = self.nested_attributes_options[association_name]
+      if attributes.respond_to?(:permitted?)
+        attributes = attributes.to_h
+      end
       attributes = attributes.with_indifferent_access
       existing_record = send(association_name)
 
diff --git a/activerecord/test/cases/nested_attributes_test.rb b/activerecord/test/cases/nested_attributes_test.rb
index d72225f3d3..933dfac806 100644
--- a/activerecord/test/cases/nested_attributes_test.rb
+++ b/activerecord/test/cases/nested_attributes_test.rb
@@ -1064,4 +1064,27 @@ class TestHasManyAutosaveAssociationWhichItselfHasAutosaveAssociations < ActiveR
     assert_not part.valid?
     assert_equal ["Ship name can't be blank"], part.errors.full_messages
   end
+
+  class ProtectedParameters
+    def initialize(hash)
+      @hash = hash
+    end
+
+    def permitted?
+      true
+    end
+
+    def to_h
+      @hash
+    end
+  end
+
+  test "strong params style objects can be assigned" do
+    params = { name: "Stern", ship_attributes:
+               ProtectedParameters.new(name: "The Black Rock") }
+    part = ShipPart.new(params)
+
+    assert_equal "Stern", part.name
+    assert_equal "The Black Rock", part.ship.name
+  end
 end