database - Android Room Persistence Library - 多对多关系

Question

我正在尝试在 Room Persistence Library 中实现多对多关系。我正在尝试创建一个Notes App with Tags.

这个想法是：

• 一个笔记会有多个标签。

• 一个标签将有多个注释。

• 在 RecyclerView 中显示所有注释以及标签。

为此，我创建了两个模型Note.java、Tag.java和TagJoin模型来存储黑白笔记和标签的关系。使用注释很容易实现一对一。@Relation

这是我的模型

@Entity(tableName = "notes")
public class Note {
    @PrimaryKey
    @NonNull
    public final String id;

    @ColumnInfo(name = "note")
    public String note;

    @Ignore
    public List<Tag> tags;

    @Ignore
    public Note(String note) {
        this(UUID.randomUUID().toString(), note);
    }

    public Note(String id, String note) {
        this.id = id;
        this.note = note;
    }

    public String getId() {
        return id;
    }

    public String getNote() {
        return note;
    }

    public void setNote(String note) {
        this.note = note;
    }

    public List<Tag> getTags() {
        return tags;
    }

    public void setTags(List<Tag> tags) {
        this.tags = tags;
    }

    @Entity(tableName = "note_tag_join",
            primaryKeys = {"noteId", "tagId"},
            foreignKeys = {
                    @ForeignKey(
                            entity = Note.class,
                            parentColumns = "id",
                            childColumns = "noteId",
                            onDelete = CASCADE),
                    @ForeignKey(
                            entity = Tag.class,
                            parentColumns = "id",
                            childColumns = "tagId",
                            onDelete = CASCADE)},
            indices = {
                    @Index(value = "noteId"),
                    @Index(value = "tagId")
            }
    )

    public static class TagJoin {
        @NonNull
        public final String noteId;
        @NonNull
        public final String tagId;

        public TagJoin(String noteId, String tagId) {
            this.noteId = noteId;
            this.tagId = tagId;
        }
    }
}

标签型号：

@Entity(tableName = "tags", indices = {@Index(value = "name", unique = true)})
public class Tag {
    @PrimaryKey
    @NonNull
    public String id;

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

    @Ignore
    public Tag(String name) {
        this(UUID.randomUUID().toString(), name);
    }

    public Tag(String id, String name) {
        this.id = id;
        this.name = name;
    }


    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

笔记道：

@Dao
public interface NoteDao {

    @Query("SELECT * FROM notes ORDER BY id DESC")
    LiveData<List<Note>> getAllNotes();

    @Insert
    long insert(Note note);

    @Update
    void update(Note note);

    @Delete
    void delete(Note note);

    @Query("DELETE FROM notes")
    void deleteAll();

    @Query("SELECT COUNT(*) FROM notes")
    int getNotesCount();

    @Query("SELECT notes.* FROM notes\n" +
            "INNER JOIN note_tag_join ON notes.id=note_tag_join.noteId\n" +
            "WHERE note_tag_join.tagId=:tagId")
    List<Note> getAllNotesOfTag(String tagId);

    @Insert
    void insert(Note.TagJoin... joins);

    @Delete
    void delete(Note.TagJoin... joins);
}

到目前为止一切都很好。现在我想在 RecyclerView 中显示笔记，但我找不到一次获取所有笔记和标签的方法。一种方法是，在 onBindViewHolder 方法中获取每个注释的标签，我认为这是错误的，因为我们必须在每次显示行时查询数据库。

请给我建议。

PS：我已经按照本文提供的代码 https://commonsware.com/AndroidArch/previews/mn-relations-in-room

标签： databasesqliteandroid-roomandroid-architecture-components