Ensure cyclic associations w/ autosave don't cause duplicate errors

This code is so fucked. Things that cause this bug not to replicate:

- Defining the validation before the association (we end up calling
  `uniq!` on the errors in the autosave validation)
- Adding `accepts_nested_attributes_for` (I have no clue why. The only
  thing it does that should affect this is adds `autosave: true` to the
  inverse reflection, and doing that manually doesn't fix this).

This solution is a hack, and I'm almost certain there's a better way to
go about it, but this shouldn't cause a huge hit on validation times,
and is the simplest way to get it done.

Fixes #20874.

5 files changed, 37 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 47364325ea..997abcb48d 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Ensure that cyclic associations with autosave don't cause duplicate errors
+    to be added to the parent record.
+
+    Fixes #20874.
+
+    *Sean Griffin*
+
 *   Ensure that `ActionController::Parameters` can still be passed to nested
     attributes.
 
diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
index 0792d19c3e..5f38bd51f6 100644
--- a/activerecord/lib/active_record/autosave_association.rb
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -222,6 +222,7 @@ module ActiveRecord
               true
             end
             validate validation_method
+            after_validation :_ensure_no_duplicate_errors
           end
         end
     end
@@ -456,5 +457,11 @@ module ActiveRecord
           end
         end
       end
+
+      def _ensure_no_duplicate_errors
+        errors.messages.each_key do |attribute|
+          errors[attribute].uniq!
+        end
+      end
   end
 end
diff --git a/activerecord/test/cases/autosave_association_test.rb b/activerecord/test/cases/autosave_association_test.rb
index 676b29ed7c..6b69b96e21 100644
--- a/activerecord/test/cases/autosave_association_test.rb
+++ b/activerecord/test/cases/autosave_association_test.rb
@@ -67,6 +67,14 @@ class TestAutosaveAssociationsInGeneral < ActiveRecord::TestCase
     assert_no_difference_when_adding_callbacks_twice_for Pirate, :parrots
   end
 
+  def test_cyclic_autosaves_do_not_add_multiple_validations
+    ship = ShipWithoutNestedAttributes.new
+    ship.prisoners.build
+
+    assert_not ship.valid?
+    assert_equal 1, ship.errors[:name].length
+  end
+
   private
 
   def assert_no_difference_when_adding_callbacks_twice_for(model, association_name)
diff --git a/activerecord/test/models/ship.rb b/activerecord/test/models/ship.rb
index 312caef604..fcc533380b 100644
--- a/activerecord/test/models/ship.rb
+++ b/activerecord/test/models/ship.rb
@@ -19,6 +19,17 @@ class Ship < ActiveRecord::Base
   end
 end
 
+class ShipWithoutNestedAttributes < ActiveRecord::Base
+  self.table_name = "ships"
+  has_many :prisoners, inverse_of: :ship, foreign_key: :ship_id
+
+  validates :name, presence: true
+end
+
+class Prisoner < ActiveRecord::Base
+  belongs_to :ship, autosave: true, class_name: "ShipWithoutNestedAttributes", inverse_of: :prisoners
+end
+
 class FamousShip < ActiveRecord::Base
   self.table_name = 'ships'
   belongs_to :famous_pirate
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 6872b49ad9..2fc806f181 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -678,6 +678,10 @@ ActiveRecord::Schema.define do
     t.datetime :updated_at
   end
 
+  create_table :prisoners, force: true do |t|
+    t.belongs_to :ship
+  end
+
   create_table :speedometers, force: true, id: false do |t|
     t.string :speedometer_id
     t.string :name