How to design api when models are related as 1-m chain?

Question

Django

For example, there are three models or resources,
For example: Ticket -1-m- Message -1-m- Attachment
In principle, they can be represented as separate resources and picked up separately when constructing the output.
But when creating a new ticket, you need to intercept the moment when a possible message with files was added to the database, and not just the ticket itself.
In cases with django, it turns out that post_save for the first model comes earlier than pre_save for the inline model, which is actually logical. And I can't figure out how to get around this.