Constructor creation using `@DocumentReference` SpEL referring to `self` in fails

[joeshadbolt]

Hi!

I am trying to use the @DocumentReference annotation as per guidance on modelling a One-To-Many relationship in the reference documentation:
https://docs.spring.io/spring-data/mongodb/docs/3.3.6/reference/html/#mapping-usage.document-references

However we are experiencing the following exception:
org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field '_id' cannot be found on null

Sample repository re-creating the issue:
https://github.com/joeshadbolt/spring-data-mongodb

Many thanks!

[dcebotarenco]

Had an attempt here:
joeshadbolt/spring-data-mongodb#1

Have a look if it works for you