在ROOM中联接表时，表中的字段名称相同 [英] The field's same name issue in the tables, when join tables in ROOM

问题描述

我正在为我的项目使用ROOM.但是我无法在表之间进行连接，因为表具有相同的名称字段.

例如，我的项目有树表"word"和"收藏夹"和书签"，具有相同的名称字段.

for example, my project has tree tables, "word" & "favorite" & "bookmark", which has a same name fields.

1个字

@Entity(tableName = "word")
public class Word {

    @NonNull
    @PrimaryKey(autoGenerate = true)
    private int id;

    @NonNull
    private String title;
    private String mean;
    private String pronunciation;

    // Getter and Setter
}

2-最喜欢的

@Entity(tableName = "favorite",
    foreignKeys = @ForeignKey(
            entity = Word.class,
            parentColumns = "id",
            childColumns = "word_id",
            onDelete = CASCADE,
            onUpdate = CASCADE))
public class Favorite {

    @NonNull
    @PrimaryKey
    private int word_id;
    private long time;

    // Getter and Setter
}

3-书签

     @Entity(tableName = "bookmark",
         primaryKeys = {"word_id", "category_id"},
         foreignKeys = {
            @ForeignKey(entity = Word.class,
                    parentColumns = "id",
                    childColumns = "word_id",
                    onDelete = CASCADE,
                    onUpdate = CASCADE)})
     public class Bookmark {
         @NonNull
         private int word_id;
         private long time;
         private int color;

        // Getter and Setter
}

要在这三个之间创建连接，我定义了一个新类型，称为"WordAndFavoriteAndBookmark"，并使用了"@Embedded"(如下所示). 为了解决相同的现场问题，我使用 prefix 作为 Thomas Fischer 响应

To create a join between the three, I defined a new type called "WordAndFavoriteAndBookmark", and used "@Embedded" (as shown below). and to fix the same field problem, I used prefix as a Thomas Fischer response

4- WordAndFavoriteAndBookmark

   public class WordAndFavoriteAndBookmark {
    @Embedded
    private Word word;

    @Embedded(prefix = "favorite_")
    private Favorite favorite;

    @Embedded(prefix = "bookmark_")
    private Bookmark bookmark;

    //Getter and Setter
    public Word getWord() { return word; }
    public void setWord(Word word) {this.word = word;}    
    public Favorite getFavorite() { return favorite; }    
    public void setFavorite(Favorite favorite) { this.favorite = favorite;}    
    public Bookmark getBookmark() { return bookmark;  }    
    public void setBookmark(Bookmark bookmark) { this.bookmark = bookmark; }    
    }

要在这些表之间创建联接，我定义了一个新的 @Dao .

To create a join between these tables I have defined a new @Dao.

 @Dao
public interface WordAndFavoriteAndBookmarkDao {
    @Query("SELECT word.*, favorite.time FROM word LEFT JOIN favorite " +
            "ON word.id = favorite.word_id")
    LiveData<List<WordAndFavoriteAndBookmark>> getAllWordsByFavoritesForLanguage();
    }

但是，再次使用查询后，在我的视图(活动性或片段)中遇到了以下代码的错误:

But again, after using the query, I encounter the error for the following code in my view(Activity or Fragment):

mWordAndFavoriteAndBookmark.getFavorite().getTime();

我认为这是由于使用了 Perfix ，但我不知道解决该问题的方法

I think this is due to the use of Perfix, but I do not know the solution to this problem

由 Thomas Fisher 回答， 一切都好.但是在查询中使用计数"或总和"时，我在读取这些值时遇到了问题，而且我不知道如何读取它们.

EDITED: by Thomas Fisher answer, everything is fine. But when used "Count" or "Sum" in my query, I have problem for reading these values and I don't know how to read them.

推荐答案

为解决"Count"或"Sum"字段问题，我使用了新的复合数据类. 如下:

To solve the "Count" or "Sum" field problem, I used the new compound data class. As below:

public static class CategoryAndEssentials {
    @Embedded
    private BookmarkCategory bookmarkCategory;

    private int wordCount;

    public BookmarkCategory getBookmarkCategory() { return bookmarkCategory; }
    public void setBookmarkCategory(BookmarkCategory bookmarkCategory) { this.bookmarkCategory = bookmarkCategory;}
    public int getWordCount() { return wordCount; }
    public void setWordCount(int wordCount) { this.wordCount = wordCount; }
}

在查询定义的Dao类中，我引用了该字段:

And in the query definition, in Dao class, I referred to this field:

@Query("SELECT " +
            "bookmarkCategory.*, " +
            "COUNT(bookmark.category_id) AS wordCount, " +
        "FROM bookmarkCategory LEFT JOIN bookmark " +
        "ON bookmarkCategory.id = bookmark.category_id " +
        "GROUP BY bookmarkCategory.id ")
LiveData<List<EntityClassForJoinTables.CategoryAndEssentials>> getAllCategoriesAndEssentials();

请注意，查询中的别名列必须与复合数据类中定义的变量相同

Note that the alias column in query must have a same as the variable defined in the compound data class

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