How to set a condition for m2m_changed to be executed?

Question

Django

Tell me, please, how can I do the following:
Models: Service, Request, Rzi, RziRequest
In the Service model - there are contracts
In the Request model - there are calls under contracts
In Rzi, RziRequest - in fact, everything is the same as above.
The Rzi model has a contract number field ,

• - if it is filled (i.e. there is a contract number), then all values ​​selected in ManyToManyField must be written to another intermediate table Service_Request
  
• - otherwise write everything to the intermediate table Rzi_RziRequest
  

Now all the selected data from the ManyToManyField is written to the intermediate table Rzi_RziRequest
How can I do this?

Answer 1

It is enough to make a method for the model, from which to return bool if

there is a contract number field,