Rails中的Active Record和设置关系的问题

I currently have a controller in rails, where i create a model object and set a relationship to that model object. I want that relationship as a response, but if the model object gets created within the first request, i get nil for the relationship. After the first request I'll get the right relationship as a response. Do I have to update the model object after setting a relationship, just after creating the object model?I currently have a controller in rails, where i