Android Room-查询中使用外键

Question

我有一个Component，其中包含来自Component_category的外键

我的目的是按照所选的组件类别过滤组件列表。

这两个类如下：

---

组件

@Entity(tableName = "component_table", foreignKeys = {
        @ForeignKey(
            entity = Rack.class,
            parentColumns = "rack_id",
            childColumns = "component_id",
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE
        ),
        @ForeignKey(
            entity = ComponentCat.class,
            parentColumns = "component_cat_id",
            childColumns = "component_id",
            onDelete = ForeignKey.CASCADE,
            onUpdate = ForeignKey.CASCADE
        )
})

public class Component {

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "component_id")
    public int componentID;

    @NonNull
    @ColumnInfo(name = "component_name")
    private String componentName;

    // .. omitted

---

组件类别

@Entity(tableName = "component_cat_table")

public class ComponentCat
{
    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "component_cat_id")
    public int componentCatID;

    @NonNull
    @ColumnInfo(name = "component_cat_name")
    private String componentCatName;

    // .. omitted

---

查询

我希望查询如下：

 @Query("SELECT * from component_table " +
        "INNER JOIN component_cat_table " +
        "WHERE component_table.component_cat_id == :categoryID ORDER BY component_name ASC")
        LiveData<List<Component>> getFilteredComponents(int categoryID);

但是在编译时它告诉我它无法解析component_table.component_cat_id。我也无法在我的Component类的外键实体中设置名称。我尝试了一些选项，但在不“访问”外键的情况下不知道如何解决查询。

Answer 1

我碰巧发现答案隐藏在一个名为“ android persistence”的示例项目中。

要使用外键，还必须定义指定外键的列。列与外键数组之间的“链接”将自动建立。

   @Entity(foreignKeys = {
        @ForeignKey(entity = Book.class,
                parentColumns = "id",
                childColumns = "book_id"),

        @ForeignKey(entity = User.class,
                parentColumns = "id",
                childColumns = "user_id")})
@TypeConverters(DateConverter.class)
public class Loan {
    @PrimaryKey
    @NonNull
    public String id;

    public Date startTime;

    public Date endTime;

    @ColumnInfo(name="book_id")
    public String bookId;

    @ColumnInfo(name="user_id")
    public String userId;
}

以上示例来自以下链接。

https://github.com/googlecodelabs/android-persistence/blob/master/app/src/main/java/com/example/android/persistence/codelab/db/Loan.java