Aha. I see what the problem could be. Your new method is trying to load a set of params, which is not the way this is usually done. Traditionally, the #new method is a GET request, which loads a generic form built around an empty new object. Since you are trying to set up the relationship with the commentable polymorphic object, maybe you should use a nested route to send that to the controller, or have a separate strong parameters accessor just for this form. You're sending the parameters as bare querystring options in the link helper, not nesting them into the comment object, which is why your accessor is failing you. If you changed the link to be
new_comment_path( 'comment[commentable_type]': 'post', 'comment[commentable_id]:
commendable.id)
...then your existing accessor will work, but that's a lot of typing when you could probably do this a lot easier with nested routes.
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