Room SQL:在两个表上都使用WHERE参数来查询具有一对多关系的对象

问题描述

我有这些课程:

@Entity
public class Person {
    long id;
    String name;
}


@Entity
public class Dog {
    long id;
    String color;

    long idPerson;
}


public class PersonWithDog {
    @Embedded
    Person person;

    @Relation(parentColumn = "id", entityColumn = "idPerson", entity =     Dog.class)
    List<Dog> dogs;
}

我想查询一个人并返回他拥有的只黑狗的列表.像这样:

I want to make a query to return a person and a list of only black dogs he owns. Something like:

SELECT * FROM Person 
LEFT JOIN Dogs ON Person.id = Dogs.idPerson 
WHERE Person.id = ? AND Dogs.color = black

使用Room可以吗?

**注意:如果我这样制作POJO:

**Note: If I make a POJO this way:

public class PersonWithDog {
    @Embedded
    Person person;

    @Embedded
    List<Dog> dogs;
}

并使用上面的查询，Room不会找出如何映射List的字段，因为它不接受嵌入式列表...

and use the above query, Room won't find out how to map the fields of List, as it doesn't accept an embedded list...

推荐答案

如果没有其他效果，此肮脏的解决方法可能会在最后帮助您.请注意，返回类型不能为 LiveData ，并且每次需要数据时都应调用该方法(并且可能将其结果包装在 SingleLiveEvent 之类的东西:

If nothing else worked, this dirty solution may help you as a last resort. Note that the return type could not be LiveData and you should call the method every time you need the data (and perhaps wrap its result in a SingleLiveEvent or something):

@Dao
public abstract class MyDao {

    // Call this method
    @Transaction
    Map<Person, List<Dog>> getPersonBlackDogs(long personId) {
        Person person = getPerson(personId);
        List<Dog> blackDogs = getBlackDogs(personId);
        return Collections.singletonMap(person, blackDogs);
    }

    // You do not want to expose these two methods so make theme protected

    @Query("SELECT * FROM Person WHERE id = :personId")
    protected abstract Person getPerson(long personId);

    @Query("SELECT * FROM Dog WHERE idPerson = :personId AND Dog.color = black")
    protected abstract List<Dog> getBlackDogs(long personId);
}

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