Question

I have a Java model similar to:

public class Country {
    @Id private String id;
    private CurrencyId currencyId;
    private List<Province> provinceList;
    ...
}

public class Province {
    @Id private String id;
    private Gobernor gobernorId;
    private List<City> cityList;
    ...
}

public class City {
    @Id private String id;
    private String name;
    ...
}

I want to store that data using objectify. However, as Country data might change, I also want to store the date the Country data has been stored, so I think I should store an entity such as:

public class CountryListEntity {
    @Id private String id;
    private List<Country> countryList;
    private Date storeDate;
}

Note I will only have one entity of kind CountryListEntity with the Id “root”, if I can store it like that. I know very little about both how google apps stores data and how objectify works. I’ve tried many combinations of @Embedded, but I got many errors, i.e.

Cannot place array or collection properties inside @Embedded arrays or collections

Can anyone tell me how to define these classes? A snippet of the code needed to store and retrieve this “root” entity, would be highly appreciated!

Answer 1

@Embedded collections are transformed into a series of collection fields in the
low-level Entity. That’s why one level embedding is all you can do.

If you are going to store/load all data at once and if your entities are as simple as the ones in your example you can put @Serialized annotation for your lists inside @Embedded lists.

You can find out more from this discussion.

The problem with this approach is that your low-level embeddings won’t be able to be indexed.

public class CountryListEntity {
    @Id private String id;

    @Embedded 
    private List<Country> countryList;

    private Date storeDate;
}

public class Country implements Serializable {
    private String id;
    private CurrencyId currencyId;

    @Serialized
    private List<Province> provinceList;
    // ...
}

public class Province implements Serializable {
    private String id;
    private Gobernor gobernorId;

    @Serialized
    private List<City> cityList;
    // ...
}

public class City implements Serializable {
    private String id;
    private String name;
    // ...
}