You need to set the :inverse_of option manually for relations that have an association to a polymorphic model. Otherwise you will not be able to save a record with a nested polymorphic association.

class Event < ApplicationRecord
  has_many :letters, as: :record
end

class Letter < ApplicationRecord
  belongs_to :record, polymorphic: true
end

event = Event.new.letters.build
event.save! # => ActiveRecord::RecordInvalid: Validation failed: Record must exist

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Using the :inverse_of option here will fix the problem.

class Event < ApplicationRecord
  has_many :letters, as: :record, inverse_of: :record
end

Note: It is not possible and necessary to set :inverse_of in the polymorphic model.