Value change in a field should not affect the value in the reference table

[Kairavi Bajaj]

Hi, I have a use case where I am populating a field "code" based on another field "city". The city is referencing the location table. Each city has its own unique code except for 3 cities which have multiple codes but the location table only stores unique value. In my form, I need to add the functionality that the default code from the table auto populates but it should also allow the end user to be able to enter another value into the field, which could be the secondary code and this change should not affect the referenced location table. 
How can I achieve this?

[Murthy Ch]

@Kairavi Bajaj 

Are you writing this on record producer. If yes, you can ignore the value to don't update into the location record.

Need more details on your requirement.