Android Room-使用适配器和回收查看器在多对多关系中显示两个表中的数据的问题

Question

我目前有两个类Recipes和Ingredients，用户在其中输入配方和与该配方相关的配料。我正在确保每一种配料都是数据库中唯一的，因此需要与我的食谱和配料建立多对多的关系。

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。。。

配方类别：

`@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;
}
}

交叉引用和从食谱中获取成分的类：

RecipeIngredientsRef类：

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

RecipeIngredients加入课程：

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DAO:

bothDAO:

@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

您似乎滥用了多对多关系。这个查询就足够了：

@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RecipeIngredientsRef表，应该同时包含配方和配料的外键。