Cast jsonb foreign key on Rails association

Using Rails 6, I'm trying to set a has_many association with a join to a postgres jsonb column.

class Form
  has_many :automation_triggers, foreign_key: "preferences->>'form_id'"
end

This results in the error:

ActiveRecord::StatementInvalid (PG::UndefinedFunction: ERROR: operator does not exist: bigint = jsonb)

The foreign key needs be cast as an integer to resolve the error so the query should look like:

SELECT "automation_triggers".* FROM "automation_triggers" INNER JOIN forms ON (forms.id = (automation_triggers.preferences->'form_id')::int)

Is there a way to apply a cast to a has_many association foreign key? or write a custom join query for the association?