Spring Data how to properly setup/save an entity?

Question

Java

@Entity
public class ReportPeriod {
    ...
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "reportPeriod")
    private Collection<Document> documents;
}

@Entity
public class Document {
    ...
    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "report_period_id", referencedColumnName = "id", nullable = false)
    private ReportPeriod reportPeriod;
}

...
ReportPeriod rp = new ReportPeriod();
rp.setDatePeriod('01.01.2001');
reportPeriodRepository.save(rp);
...

When creating a new period, I get:
rp.Id = 123
rp.datePeriod = '01/01/2001'
rp.documents = null
Question. Why after saving the period I get that documents == null? In theory, an empty collection should return?
If you search for another previously saved period: reportPeriodRepository.findOne(345). Then rp.getDocuments() returns a collection and not null.

Answer 1

The collection will be empty if the object is loaded from the database. If the object was created in code and saved to the database, then Hibernate will not initialize anything.
I usually use the following code for internal collection getters (I peeped from JAXB):

public Collection<Document> getDocuments() {
    if (documents == null) {
        documents = new ArrayList<>();
    }
    return documents;
}

Answer 2

rp = reportPeriodRepository.save(rp);
And you will be happy. But it's better to create an empty collection in the get method if needed. In general, it's better to stop using @OneToMany and @ManyToMany collections.