无法将计数值设置为列

Question

我正在尝试获取在机架中具有一定数量组件的组件类别。

查询

@Query("SELECT id, name, component_amount = " +
        "       (SELECT COUNT(component.id) as amount " +
        "        FROM component " +
        "        WHERE component.rack_id = :rackID AND component_cat.id = component_cat_id ) " +
        "FROM component_cat")
LiveData<List<ComponentCat>> getRackComponentCategories(long rackID);

---

组件猫

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

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

    @ColumnInfo(name = "component_amount")
    private int amountOfComponents;


    public ComponentCat(String componentCatName, int amountOfComponents) {
        this.componentCatName = componentCatName;
        this.amountOfComponents = amountOfComponents;
    }

---

在 PHPMyAdmin 中测试它可以工作，但是将此 SQL 放在我的 Room 应用程序中，我收到以下错误：

  error: The columns returned by the query does not have the fields [amountOfComponents] in com.mwb.digitalstorage.model.ComponentCat even though they are annotated as non-null or primitive. Columns returned by the query: [id,name,component_amount =        (SELECT COUNT(component.id) as amount         FROM component         WHERE component.rack_id = :rackID AND component_cat.id = component_cat_id )]

在 Room 中，如何将变量 amountOfComponents 设置为 COUNT() 结果？

我试过了：

• 有一个 COUNT(component.id) 作为 component_amount，但这样不会给出正确的结果
• 在 MySQL 中重新创建表并执行相同的查询，结果确实正确
• 在字段中删除@NonNull，没有区别

• 分配component_amount.amount

  @Query("SELECT id, name, component_amount.amount = " + " (SELECT COUNT(component.id) as amount " ...

  这不会构建

Answer 1

Room 不支持为您的列指定别名。

您的错误消息指出：

Columns returned by the query: [id,name,component_amount =        (SELECT COUNT(component.id) as amount         FROM component         WHERE component.rack_id = :rackID AND component_cat.id = component_cat_id )

这意味着它将整个查询解释为一列。

尝试将您的查询更改为以下内容：

@Query("SELECT id, name, " +
        "       (SELECT COUNT(component.id) as amount " +
        "        FROM component " +
        "        WHERE component.rack_id = :rackID AND component_cat.id = component_cat_id ) as component_amount " +
        "FROM component_cat")
LiveData<List<ComponentCat>> getRackComponentCategories(long rackID);