Rollback parent transaction when children fails to update (#32796)

* Rollback parent transaction when children fails to update

Rails supports autosave associations on the owner of a `has_many`
relationship. In certain situation, if the children of the association
fail to save, the parent is not rolled back.

```ruby
class Employee < ActiveRecord::Base
end

class Company < ActiveRecord::Base
  has_many(:employees)
end

company = Company.new
employee = company.employees.new
company.save
```

In the previous example, if the Employee failed to save, the Company
will not be rolled back. It will remain in the database with no
associated Employee.

I expect the `company.save` call to be atomic, and either create all or
none of the records.

The persistance of the Company already starts a transaction that nests
it's children. However, it didn't track the success or failure of it's
children in this very situation, and the outermost transaction is not
rolled back.

This PR makes the change to track the success of the child insertion and
rollback the parent if any of the children fail.

* Change the test to reflect what we expect

Once #32862 is merged, rolling back a record will rollback it's state to match
the state before the database changes were applied

* Use only the public API to express the tests

* Refactor to avoid reassigning saved for nested reflections

[Guillaume Malette + Rafael Mendonça França]

activerecord/lib/active_record/autosave_association.rb

@@ -400,8 +400,11 @@ def save_collection_association(reflection)
               if autosave != false && (@new_record_before_save || record.new_record?)
                 if autosave
                   saved = association.insert_record(record, false)
-                else
-                  association.insert_record(record) unless reflection.nested?
+                elsif !reflection.nested?
+                  association_saved = association.insert_record(record)
+                  if reflection.validate?
+                    saved = association_saved
+                  end
                 end
               elsif autosave
                 saved = record.save(validate: false)

activerecord/test/cases/autosave_association_test.rb

@@ -517,6 +517,18 @@ def test_invalid_adding_before_save
     assert_not_predicate new_firm, :persisted?
   end
 
+  def test_adding_unsavable_association
+    new_firm = Firm.new("name" => "A New Firm, Inc")
+    client = new_firm.clients.new("name" => "Apple")
+    client.throw_on_save = true
+
+    assert_predicate client, :valid?
+    assert_predicate new_firm, :valid?
+    assert_not new_firm.save
+    assert_not_predicate new_firm, :persisted?
+    assert_not_predicate client, :persisted?
+  end
+
   def test_invalid_adding_with_validate_false
     firm = Firm.first
     client = Client.new

activerecord/test/models/company.rb

@@ -145,6 +145,11 @@ class RaisedOnSave < RuntimeError; end
     raise RaisedOnSave if raise_on_save
   end
 
+  attr_accessor :throw_on_save
+  before_save do
+    throw :abort if throw_on_save
+  end
+
   class RaisedOnDestroy < RuntimeError; end
   attr_accessor :raise_on_destroy
   before_destroy do