"Unsupported argument type: NilClass. Construct an Arel node instead." when using where(polymorphic_association: [])

[jalyna]

First of all thanks for the great work! I hope I thoroughly checked all created issues so this is no duplicate.

Steps to reproduce

I created a minimal repository with rails new :

• https://github.com/jalyna/demo-polymorphic-rails61-issue (using rails 6.1.1, error also appears on 6.1.0)

See minimal changes in this commit:
jalyna/demo-polymorphic-rails61-issue@928fd7b

Basically this test

assert_nil Subscription.find_by(plan: []) # plan is a polymorphic belongs_to association

fails with 6.1.x, resulting in: Unsupported argument type: NilClass. Construct an Arel node instead.

I also created a branch using 6.0.3 where the test passes. https://github.com/jalyna/demo-polymorphic-rails61-issue/tree/rails60

This test will pass though (no rows within public_plans):

assert_nil Subscription.find_by(plan: PublicPlan.all)

The workaround I'm currently using in my projects is to not trigger a query when the array is empty.

Expected behavior

Subscription.find_by(plan: []) # plan is a polymorphic belongs_to association

should return nil

Actual behavior

Subscription.find_by(plan: []) # plan is a polymorphic belongs_to association

raises: Unsupported argument type: NilClass. Construct an Arel node instead.

System configuration

Rails version: 6.1.0 & 6.1.1

Ruby version: 2.7.1

[dbussink]

This was fixed with #41063, the same issue was identified in #40951 (comment). There's no release yet though afaik that includes this fix.

[jalyna]

@dbussink thank you!