Android Room 加入的返回类型 [英] Return type for Android Room joins

问题描述

假设我想在两个实体 Foo 和 Bar 之间做一个 INNER JOIN:

Let's say I want to do an INNER JOIN between two entities Foo and Bar:

@Query("SELECT * FROM Foo INNER JOIN Bar ON Foo.bar = Bar.id")
List<FooAndBar> findAllFooAndBar();

是否可以强制使用这样的返回类型?

Is it possible to force a return type like this?

public class FooAndBar {
    Foo foo;
    Bar bar;
}

当我尝试这样做时，我收到此错误:

When I try to do that, I get this error:

error: Cannot figure out how to read this field from a cursor.

我也试过给表名加上别名以匹配字段名，但这也不起作用.

I've also tried aliasing the table names to match the field names, but that didn't work either.

如果这是不可能的，我应该如何干净地构建一个兼容的返回类型，其中包含两个实体的所有字段?

If this isn't possible, how should I cleanly construct a compatible return type that includes all fields for both entities?

推荐答案

道

@Query("SELECT * FROM Foo")
List<FooAndBar> findAllFooAndBar();

类 FooAndBar

public class FooAndBar {
    @Embedded
    Foo foo;

    @Relation(parentColumn =  "Foo.bar_id", entityColumn = "Bar.id")
    List<Bar> bar;
    // If we are sure it returns only one entry
    // Bar bar;

    //Getter and setter...
}

这个解决方案似乎有效，但我并不为此感到自豪.你怎么看?

This solution seems to work, but I'm not very proud of it. What do you think about it?

另一种解决方案

Dao，我更喜欢明确选择，但*"会做这个工作:)

Dao, I prefer to explicitly select but "*" will do the job :)

@Query("SELECT Foo.*, Bar.* FROM Foo INNER JOIN Bar ON Foo.bar = Bar.id")
List<FooAndBar> findAllFooAndBar();

类 FooAndBar

public class FooAndBar {
    @Embedded
    Foo foo;

    @Embedded
    Bar bar;

    //Getter and setter...
}

从2.2.0-alpha01版本开始，房间@Relation注解可以管理一对一关系

edit: since Version 2.2.0-alpha01, room @Relation annotation can manage One-To-One relation

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