Develop Reference

One-to-many relation in Room

I've been using SugarDB for most of my projects in the past. It was easy to use and satisfied most of my requirements but since that project has been abandoned, decided to look at alternatives and Room seems like the best option.
However, some basic things are quite confusing in Room. My Object uses Gson to populate data from a webservice, and as such as links to other objects. As an example, consider the classes below:
#Entity
public class TestModel
{
#PrimaryKey(autoGenerate = true)
private int id;
private String name;
private String age;
private List<Book> issuedBooks;
}
public class Book
{
private String title;
private int ISBN;
}
Now if my first class is annotated as the Entity, will this automatically treat classes referenced inside it as entities as well?
If I save an object of TestModel, will it save the list of Books with it to the database?

I guess you can do it this way.
#Entity
public class TestModel {
#PrimaryKey
public int id; // TestModel id
public String name;
public String age;
}
#Entity
public class Book {
#PrimaryKey
public int id; // Book id
public int testModelId; // TestModel id
public String title;
public int ISBN;
}
public class TestModelWithBooks {
#Embedded
public TestModel testModel;
#Relation(parentColumn = "id", entityColumn = "testModelId", entity = Book.class)
public List<Book> books;
}
For their Dao, you can write it this way.
#Dao
public interface TestModelDao {
#Query("SELECT * FROM TestModel")
public List<TestModelWithBooks> loadTestModelsWithBooks();
}

will this automatically treat classes referenced inside it as entities as well?
No. In fact, I would expect your code to fail to compile. You would need to:
Make Book be an #Entity
Remove issuedBooks from TestModel
Set up a #ForeignKey relationship between Book and TestModel
If I save an object of TestModel, will it save the list of Books with it to the database?
No.
Room is not an ORM. Room is a thin object wrapper around SQLite. #Entity and #ForeignKey model the table structure. IMHO, the simplest way to think of Room as it being DTOs to the database. Your model objects that represent your object graph are not the entities, but instead are built from the entities. This is akin to how responses from a Web service (e.g., Retrofit) are DTOs to the server, and you may need to map from those objects to the "real" model objects that you want to use in the app.

How to handle relationships in Room?

I'm testing Room persistence library. I have two entity classes:
#Entity
public class User {
#PrimaryKey
int id;
String name;
}
#Entity(foreignKeys = #ForeignKey(entity = User.class,
parentColumns = "id",
childColumns = "userId")
public class Pet {
#PrimaryKey
int id;
String name;
int userId;
}
Now I'd like to get list of Pet and in each Pet object keep actual reference to User according to his userId. So each time when the userId is changed, this reference also should be changed. Is it possible to do? Or maybe there is a better way to handle relation like this?

Actually, ROOM not recommended to do that. The reasons are in the reference link.
Maybe you could try it,
#Entity
public class User {
#PrimaryKey
int id;
String name;
#Embedded
public List<Pet> userPet;
}
public class Pet {
int id;
String name;
}
Understand why Room doesn't allow object references
A convenience annotation which can be used in a Pojo to automatically fetch relation entities.

How to handle Object References in Entity?

I have the following structure to save to app database:
#Entity
public class Project{
#primaryKey
String id;
String name;
[...]
Country country;
[...]
}
And my Country Entity looks like the following:
#Entity
public class Country {
#PrimaryKey
private String id;
private String name;
private String pk;
}
Now to my Question: How do I make Room know the Relation between Country and Project Entity?

Room can not have nested entities, you can embedd POJO classes in an entity but it will get flattened into a single table or if you want Country as an entity then you'll have to store county_id in Project entity and index it as foreign key.
More on Embedded fields: https://developer.android.com/reference/android/arch/persistence/room/Embedded.html
More on Foreign key: https://developer.android.com/reference/android/arch/persistence/room/ForeignKey.html

Please refer the official documentation
https://developer.android.com/reference/android/arch/persistence/room/Relation.html

Query based on RealmList as member variable

How can we query Realm Database based on the RealmList as the member variable?
Let me further clarify the use case.
I have the following class:
public class Match extends RealmObject {
#PrimaryKey
#Expose
private String matchid;
#Expose
private RealmList<Team> Team = new RealmList<>();
}
And Team object looks like this:
public class Team extends RealmObject {
#PrimaryKey
#Expose
private String teamid;
}
I want to query all Match Objects in which teamid is 1 for example.
Is it possible? How can we write such query in Realm?

RealmResults<Match> results = realm.where(Match.class)
.equalTo("Team.teamid", 1).findAll();
More info on link queries: https://realm.io/docs/java/latest/#link-queries

Manually add #Required to the primary field to maintain the same behavior as 0.88.3 (upgrade to 0.89.0)

I just updated realm from 0.88.3 to 0.89.0. In the changelog it says
PrimaryKey fields are no longer automatically marked as #Required. They can now be null if the type of the field can usually be null.
This change will throw a RealmMigrationNeededException. Either manually add #Required to the primary field to maintain the same behavior as 0.88.3 and below, or change the nullability in a migration step.
What I tried:
1.
Add #Required to the object but it throws "Error:(542, 114) error: incompatible types: String cannot be converted to long"
public class Migration{
.
.
.
schema.get("Retailer")
.setRequired("id", true);
.
.
}
#RealmClass
public class Retailer extends RealmObject {
#PrimaryKey #Required
private String id;
.
.
.
}
How to do it correct?

Thanks to beeender, I updated to the newer version 0.89.1 and it is working now.
Just add #Required to you existing Primary key and no migration is needed.
#RealmClass
public class Retailer extends RealmObject {
#PrimaryKey #Required
private String id;
}
If you add a new RealmObject to your project you have to write the following migration:
schema.create("Retailer")
.addField("id", String.class, FieldAttribute.PRIMARY_KEY, FieldAttribute.REQUIRED)