java Android Room问题使用适配器和循环查看器以多对多关系显示两个表中的数据

Question

我目前有两个类Recipes and Components，用户在其中输入一个配方和与该配方配套的配料。我要确保数据库中的每种配料都是唯一的，因此需要与我的食谱和配料建立多对多关系

然而，我的问题是，我只能显示配方类中的值，当我打印配料时，它是空的

食谱类：

`@Entity(tableName = "recipe_table")
    public class Recipe {
    @PrimaryKey(autoGenerate = true)
    public Integer recipeId;
    @NonNull
    public String title;
    @NonNull
    public String instructions;
    public Integer rating;
    
    public Recipe( @NonNull String title, @NonNull String instructions, Integer rating) {
        this.title = title;
        this.instructions = instructions;
        this.rating = rating;
    }

    public Integer getId(){
        return recipeId;
    }
    public String getTitle() {
        return title;
    }
    public String getInstructions() {
        return instructions;
    }
    public Integer getRating() {
        return rating;
    }

    }`

成分类别：

@Entity(tableName = "ingredients_table")
public class Ingredients {

    @PrimaryKey(autoGenerate = true)

    @NonNull
    public Integer ingredientId;
    @NonNull
    public String ingredients;


    public Ingredients(@NonNull String ingredients) {
        this.ingredients = ingredients;
    }

    public String getIngredients(){
        return ingredients;
    }

}

交叉参考并从食谱中获取配料的课程：

ReceiveCreditsRef类别：

@Entity(tableName = "recipe_ingredients_table", primaryKeys = {"recipeId", "ingredientId"})
public class RecipeIngredientsRef {
    @ColumnInfo @NonNull
    public Integer ingredientId;
    @ColumnInfo @NonNull
    public Integer recipeId;
}

RecipeIngredientsJoin类：

public class RecipeIngredientsJoin{

    @Embedded public Recipe recipe;
    @Relation(
            parentColumn = "recipeId",
            entityColumn =  "ingredientId",
            associateBy = @Junction(RecipeIngredientsRef.class)
    )
public List<Ingredients> ingredients;

    public List<Ingredients> getIngredients() {
        return ingredients;
    }
}

还有我的刀：

博思道：

@Dao
public interface bothDAO {

    @Transaction
    @Query("SELECT *, * FROM recipe_table INNER JOIN ingredients_table ON recipeId = ingredientId")
    public LiveData<List<RecipeIngredientsJoin>> getAllIngredientsFromRecipe();

我使用这个DAO是为了显示我的项目，如果你需要查看我的数据库类来帮助解决这个问题，请告诉我

Answer 1

你似乎误用了多对多的关系。 这个问题就足够了：

@Transaction
@Query("SELECT * FROM recipe_table") // < - no need to JOIN, Room do it for you
// In addition, condition `INNER JOIN ingredients_table ON recipeId = ingredientId` is wrong, 
// since you're trying to match ingredients & recipes ids 
public LiveData<List<RecipeIngredientsJoin>> getAllIngredientsFromRecipe();

此方法应该返回您想要的内容。当然，对于这个RecipeIngredientsRef表应该包含食谱和配料的外键