Polymorphic has_one touch: Reset association cache result after create transaction

In case of a polymorphic association there's no automatic inverse_of to assign the
inverse record. So to get the record there needs to be a query executed,
however, if the query fires within the transaction that's trying to create
the associated record, no record can be found. And worse, the nil result is cached
on the association so after the transaction commits the record can't be found.

That's what happens if touch is enabled on a polymorphic has_one association.

Consider a Comment with a commentable association that needs to be touched.

For `Comment.create(commentable: Post.new)`, the existing code essentially
does `commentable.send(:comment)` within the create transaction for the comment
and thus not finding the comment.

Now we're purposefully clearing the cache in case we've tried accessing
the association within the transaction and found no object.

Before:

```
kaspth-imac 2.6.3 ~/code/rails/activerecord master *= ARCONN=postgresql bin/test test/cases/associations/has_one_associations_test.rb -n /commit/
Using postgresql
Run options: -n /commit/ --seed 46022

D, [2019-07-19T03:30:37.864537 #96022] DEBUG -- :   Chef Load (0.2ms)  SELECT "chefs".* FROM "chefs" WHERE "chefs"."employable_id" = $1 AND "chefs"."employable_type" = $2 LIMIT $3  [["employable_id", 1], ["employable_type", "DrinkDesignerWithPolymorphicTouchChef"], ["LIMIT", 1]]
D, [2019-07-19T03:30:37.865013 #96022] DEBUG -- :   Chef Create (0.2ms)  INSERT INTO "chefs" ("employable_id", "employable_type") VALUES ($1, $2) RETURNING "id"  [["employable_id", 1], ["employable_type", "DrinkDesignerWithPolymorphicTouchChef"]]
D, [2019-07-19T03:30:37.865201 #96022] DEBUG -- :   TRANSACTION (0.1ms)  RELEASE SAVEPOINT active_record_1
D, [2019-07-19T03:30:37.874136 #96022] DEBUG -- :   TRANSACTION (0.1ms)  ROLLBACK
D, [2019-07-19T03:30:37.874323 #96022] DEBUG -- :   TRANSACTION (0.1ms)  ROLLBACK
F

Failure:
HasOneAssociationsTest#test_polymorphic_has_one_with_touch_option_on_create_wont_cache_assocation_so_fetching_after_transaction_commit_works [/Users/kaspth/code/rails/activerecord/test/cases/associations/has_one_associations_test.rb:716]:
--- expected
+++ actual
@@ -1 +1 @@
-#<Chef id: 1, employable_id: 1, employable_type: "DrinkDesignerWithPolymorphicTouchChef", department_id: nil, employable_list_type: nil, employable_list_id: nil>
+nil
```

After:

```
kaspth-imac 2.6.3 ~/code/rails/activerecord master *= ARCONN=postgresql bin/test test/cases/associations/has_one_associations_test.rb -n /commit/
Using postgresql
Run options: -n /commit/ --seed 46022

D, [2019-07-19T03:30:22.479387 #95973] DEBUG -- :   Chef Create (0.3ms)  INSERT INTO "chefs" ("employable_id", "employable_type") VALUES ($1, $2) RETURNING "id"  [["employable_id", 1], ["employable_type", "DrinkDesignerWithPolymorphicTouchChef"]]
D, [2019-07-19T03:30:22.479574 #95973] DEBUG -- :   TRANSACTION (0.1ms)  RELEASE SAVEPOINT active_record_1
D, [2019-07-19T03:30:22.482051 #95973] DEBUG -- :   Chef Load (0.1ms)  SELECT "chefs".* FROM "chefs" WHERE "chefs"."employable_id" = $1 AND "chefs"."employable_type" = $2 LIMIT $3  [["employable_id", 1], ["employable_type", "DrinkDesignerWithPolymorphicTouchChef"], ["LIMIT", 1]]
D, [2019-07-19T03:30:22.482317 #95973] DEBUG -- :   TRANSACTION (0.1ms)  ROLLBACK
D, [2019-07-19T03:30:22.482437 #95973] DEBUG -- :   TRANSACTION (0.1ms)  ROLLBACK
.

Finished in 0.088498s, 11.2997 runs/s, 22.5994 assertions/s.
1 runs, 2 assertions, 0 failures, 0 errors, 0 skips
```

Notice the select now fires after the commit.

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/test/models/drink_designer.rb b/activerecord/test/models/drink_designer.rb
index 8258408f35..fe7f5c9e03 100644
--- a/activerecord/test/models/drink_designer.rb
+++ b/activerecord/test/models/drink_designer.rb
@@ -10,5 +10,11 @@ class DrinkDesignerWithPolymorphicDependentNullifyChef < ActiveRecord::Base
   has_one :chef, as: :employable, dependent: :nullify
 end
 
+class DrinkDesignerWithPolymorphicTouchChef < ActiveRecord::Base
+  self.table_name = "drink_designers"
+
+  has_one :chef, as: :employable, touch: true
+end
+
 class MocktailDesigner < DrinkDesigner
 end