How to properly apply PrePersist like logic using jpa/spring-boot - spring

Question

I have a very simple use case for the following model
#Entity
#Table(name='Foo')
class Foo {
#Id
String id = UUID.randomUUID()
String bar
Date foo_updated
}
I'd like to set the foo_updated value to new Date() when I see the incoming json payload has a value for "bar" (ie- this is a new value included in the POST/ a part of the PATCH update / included and proven to be different in a PUT)
I was hoping to simply apply the #PrePersist annotation on this model and add a simple conditional asking if "bar" was valid ...but quickly realized I wouldn't know if the value was "different" from what was in the db already (for the PATCH/PUT scenario).
I'm starting down the road of "add my own RestController" and apply this logic on the way in using the spring 4 ResponseEntity approach but ... I feel this might end up being a lot more work/more code to maintain.
As I'm new to spring-boot/spring-mvc/jpa I'm curious what other options I have and what the preferred approach would be for this seemingly "simple" requirement
Thanks for the help!

Answer 1

I came across this approach recently which involves recording the previous state on load. You now have access to the previous state after the new values are bound.
#Entity
#Table(name='Foo')
class Foo {
#Id
String id = UUID.randomUUID()
String bar
Date foo_updated
#Transient
private Foo previousState;
#PostLoad
private void setPreviousState(){
previousState = new Foo();
//copy the fields
}
}
However in your case can't you just do:
#Entity
#Table(name='Foo')
class Foo {
#Id
private String id = UUID.randomUUID()
private String bar
private Date lastUpdated;
public void setBar(String bar){
if(! this.bar.equals(bar){
lastUpdated = new Date();
}
}
}