Make field insertable but not updatable in Spring Data REST + MongoDB - jackson

Question

With Spring Data REST and Spring Data Mongo, I want to make a domain field (field username of domain User in my example) insertable when create but un-updatable when update. In other words an equivalent of JPA #Column(insertable = true, updatable = false).
I try a few approach but not work.
In my github project, domain class and repository are put in /src/main/java/*/*/User.java and UserRepository.java. The test is put in /src/test/java/*/*UserTest.java.
1. Spring Data annotation #ReadOnlyProperty and #Transient
The field is un-insertable when save to DB. See package readonlyproperty and transient_ in the project.
2. Jackson annotation #JsonProperty(access=READ_ONLY)
The field is un-insertable when create via POST request, because the JSON property is ignored when initiate an object. See package jsonpropertyreadonly in the project.
3. #JsonCreator on constructor and #JsonIgnore on setter
If the un-updatable field username is contained in json body of PUT or PATCH request, and username value changes, username get updated, which is unexpected. See package jsoncreator in the project.
4. Do not write a setter
same as 3. See package nosetter in the project.
5. Toggle on/off feature
spring.jackson.deserialization.fail-on-ignored-properties=false
spring.jackson.deserialization.fail-on-unknown-properties=false
spring.jackson.mapper.infer-property-mutators=false
not help
Spring Data REST PUT and PATCH Internal Implementation
PUT: it uses Jackson ObjectMapper.readerFor(Class) to initiate a new object
PATCH: it uses Jackson ObjectMapper.readerForUpdating(objectToUpdate).readValue(json), which use setter to update the objectToUpdate. Seems readerForUpdating doesn't see the #JsonIgnore on setter.
The only solution I know is implementing the setter in below way
void setUsername(String usernameToSet) {
if (null == this.username)
this.username = usernameToSet;
}
And disable PUT method, only use PATCH to update. See package setterchecknull.
Is there a better way? Thank you very much!!