Room - 如何从 DAO 查询中获取 COUNT 值？

Posted 2023-05-08

【中文标题】Room - 如何从 DAO 查询中获取 COUNT 值？

【英文标题】：Room - How to get COUNT value from DAO query?

【发布时间】：2019-11-12 17:09:03

【问题描述】：

我的 Room DAO 中有一个查询（对于提供的 countryId）查询我的 share_groups 表并返回包含 num_twitter_users 数字的行（通过计算相关的填充行数来计算） share_group_individuals 表）。

这是查询代码：

@Query("SELECT `share_groups`.`_id`, `share_group_name`, `share_group_description`, `share_group_instructions`, `share_group_order`, `share_group_is_visible`, `share_groups`.`last_modified`" +

    // `num_twitter_users` will contain the number of share_group_individuals within this share_group that have a Twitter username.
    ", COUNT(DISTINCT(CASE WHEN (`share_group_individuals`.`share_group_individual_twitter` IS NOT NULL AND `share_group_individuals`.`share_group_individual_twitter` != '') THEN `share_group_individuals`.`_id` ELSE NULL END)) AS `num_twitter_users`" +

    " FROM `share_groups`" +

    // Join with `share_group_individuals` - on countryId
    " LEFT JOIN `share_group_individuals` ON (`share_group_individuals`.`share_group_id` = `share_groups`.`_id`" +
    " AND `share_group_individuals`.`share_group_individual_country_id` = :countryId)" +

    // Search on countryId
    " WHERE `share_group_individuals`.`share_group_individual_country_id` = :countryId" +

    " GROUP BY `share_groups`.`share_group_name`, `share_group_individuals`.`share_group_id`" +

    // Only return share_groups that contain at least 1 share_group_individual with a Twitter account
    " HAVING `num_twitter_users` > 0" +

    " ORDER BY `share_group_order` ASC")
LiveData<List<ShareGroup>> getAllShareGroups(long countryId);

但是，我无法从返回的 LivaData<List<ShareGroup>> 访问 num_twitter_users 值。

我想知道解决方案的一部分是否可能是创建一个ShareGroupWithData POJO（因此从我的查询中返回一个LivaData<List<ShareGroupWithData>>）但是，如果这是正确的方法，我将如何填充numTwitterUsers ShareGroupWithData 类中的变量？

我在网上或official documentation 中找不到任何关于此的信息，因此非常感谢您指出正确的方向和/或一些示例代码。

为了完整起见，我的share_groups 表由该实体表示：

@Entity(tableName = "share_groups")
public class ShareGroup 

    @PrimaryKey
    @ColumnInfo(name = "_id")
    private long id;

    @NonNull
    @ColumnInfo(name = "share_group_name")
    private String shareGroupName;

    @ColumnInfo(name = "share_group_description")
    private String shareGroupDescription;

    @ColumnInfo(name = "share_group_instructions")
    private String shareGroupInstructions;

    @ColumnInfo(name = "share_group_order", defaultValue = "10")
    private int shareGroupOrder;

    @ColumnInfo(name = "share_group_is_visible", defaultValue = "1")
    private boolean shareGroupIsVisible;

    @ColumnInfo(name = "last_modified")
    private long lastModified;

...

而我的share_groups_individual 表就是由这个实体表示的：

@Entity(tableName = "share_group_individuals",
        indices = @Index(name = "index_share_group_id", value = "share_group_id"),
        foreignKeys = @ForeignKey(entity = ShareGroup.class, parentColumns = "_id", childColumns = "share_group_id", onDelete = ForeignKey.CASCADE)
)
public class ShareGroupIndividual 

    @PrimaryKey
    @ColumnInfo(name = "_id")
    private long id;

    @ColumnInfo(name = "share_group_id")
    private long shareGroupId;

    @ColumnInfo(name = "share_group_individual_country_id", defaultValue = "-1")
    private long shareGroupIndividualCountryId;

    @ColumnInfo(name = "share_group_individual_name")
    private String shareGroupIndividualName;

    @ColumnInfo(name = "share_group_individual_twitter")
    private String shareGroupIndividualTwitter;

...

【问题讨论】：

如果您为查询结果创建一个单独的类，它可能会起作用。像这样的东西：data class ResultType( @Embedded val grup: ShareGroup, @Embedded val individual: ShareGroupIndividual, val num_twitter_users: Int )

@Daniil 感谢您的帮助，但我现在已经搞定了。

【参考方案1】：

最后，我通过创建这样的 POJO 解决了这个问题：

public class ShareGroupWithData 

    @Embedded
    public ShareGroup shareGroup;

    @ColumnInfo(name = "num_twitter_users", defaultValue = "0")
    public int numTwitterUsers;
...

我的 DAO 代码更改为：

LiveData<List<ShareGroupWithData>> getAllShareGroupsWithData(long countryId);