android - 无法将计数值设置为列

我正在尝试获取一个在机架中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);

成分猫

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中测试它是有效的，但是将这个sql放到我的房间应用程序中，我得到以下错误：

  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 )]

在房间里，如何将变量amountOfComponents设置为COUNT()结果？
我试过：
有一个count（component.id）作为component_amount，但这样做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，这确实给出了正确的结果
在字段中删除@nonnull，没有区别
分配分量金额
@查询（“选择id，name，component_amount.amount=”+
“（选择count（component.id）作为amount”……
这不会建立

最佳答案

room不支持将别名提供给列的这种类型。
错误消息说明：

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个列。
尝试将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);