EF Core - 向多對多關系添加數據時出現問題

Question

在解釋我的問題之前，我只想解釋一下我的代碼目標是什么。

我的目標是創建一個簡單的桌面應用程序，用於從選定的食譜創建食譜和購物清單。 所以我希望能夠創建彼此分開的成分和食譜，然后將一定數量的成分（我稱之為 RecipeIngredients）添加到食譜中。 RecipeIngredient 類包含有關成分的信息，還包含數量和度量（例如 kg）。

話雖如此，我的問題是我一直在嘗試將 RecipeIngredients 添加到我的食譜中。

我已經使用 EF Core 建立了一個數據庫，並且能夠單獨存儲成分和食譜。 但是，當我嘗試將 RecipeIngredient 存儲在配方中時，出現以下錯誤：未處理的異常。 System.NullReferenceException：對象引用未設置為對象的實例。

我認為我的問題是我不完全了解創建RecipeIngredient 對象需要輸入哪些數據，或者如何將Recipe 鏈接到RecipeIngredient。

我已經在下面發布了我的代碼，從 main 開始。 你們中的任何人都可以指出我正確的方向嗎？ 這是我第一次使用 EF Core，也是我在 C# 中嘗試的第一個應用程序，所以我可能做錯了什么。

//Main
        static void Main(string[] args)
        {
            SQLightRecipeRepository context = new SQLightRecipeRepository();
            var recipe = context.GetRecipe(1); // I already added recipes to the database, so they exist.
            var ingredient = context.GetIngredient(1); // Already added ingredients to the database, so they exist.
            recipe.RecipeIngredients.Add(new RecipeIngredient() { Ingredient = ingredient, Measurement = Measurement.kg, Quantity = 2 });
            context.Save();
        }

//Repository
 public class SQLightRecipeRepository : IRecipeRepository, IIngredientRepository, IRecipeIngredientRepository
    {
        private RecipeContext context = new RecipeContext();

        //Recipe
        public void DeleteRecipe(int recipeId)
        {
            Recipe recipe = context.Recipes.Find(recipeId);
            context.Recipes.Remove(recipe);
        }

        public Recipe GetRecipe(int recipeId)
        {
            return context.Recipes.Find(recipeId);
        }

        public IEnumerable<Recipe> GetRecipes()
        {
            return context.Recipes.ToList();
        }

        public void InsertRecipe(Recipe recipe)
        {
            context.Recipes.Add(recipe);
        }


        public void UpdateRecipe(Recipe recipe)
        {
            context.Entry(recipe).State = Microsoft.EntityFrameworkCore.EntityState.Modified;
        }

        public IEnumerable<RecipeIngredient> GetAllIngredients(int recipeId)
        {
            return context.Recipes.Find(recipeId).RecipeIngredients.ToList();
        }


        //Ingredient operations
        public IEnumerable<Ingredient> GetIngredients()
        {
            return context.Ingredients.ToList();
        }


        public Ingredient GetIngredient(int ingredientId)
        {
            return context.Ingredients.Find(ingredientId);
        }

        public void InsertIngredient(Ingredient ingredient)
        {
            context.Ingredients.Add(ingredient);
        }

        public void DeleteIngredient(int ingredientId)
        {
            Ingredient ingredient = context.Ingredients.Find(ingredientId);
            context.Ingredients.Remove(ingredient);
        }

        public void UpdateIngredient(Ingredient ingredient)
        {
            context.Entry(ingredient).State = Microsoft.EntityFrameworkCore.EntityState.Modified;
        }


        public void Save()
        {
            context.SaveChanges();
        }

        public IEnumerable<RecipeIngredient> GetRecipeIngredients(int recipeId)
        {
            var ingredients = context.RecipeIngredients.Where(r => r.RecipeId == recipeId);
            return ingredients.ToList();
        }

        public IEnumerable<RecipeIngredient> GetRecipeIngredients()
        {
            return context.RecipeIngredients;
        }

        public RecipeIngredient GetRecipeIngredient(int ingredientId, int recipeId)
        {
            return context.RecipeIngredients.Find(recipeId, ingredientId); 
        }

        public void InsertRecipeIngredient(RecipeIngredient recipeIngredient)
        {
            context.RecipeIngredients.Add(recipeIngredient);
        }

        public void DeleteRecipeIngredient(RecipeIngredient recipeIngredient)
        {
            context.RecipeIngredients.Remove(recipeIngredient);
        }

        public void UpdateRecipeIngredient(RecipeIngredient recipeIngredient)
        {
            context.Entry(recipeIngredient).State = Microsoft.EntityFrameworkCore.EntityState.Modified;
        }

//DBContext (RecipeContext)
    public class RecipeContext : DbContext
    {
        public DbSet<Recipe> Recipes { get; set; }
        public DbSet<Ingredient> Ingredients { get; set; }
        
        public DbSet<RecipeIngredient> RecipeIngredients { get; set; }

        public string DbPath { get; }

        public RecipeContext()
        {
            var folder = Environment.SpecialFolder.LocalApplicationData;
            var path = Environment.GetFolderPath(folder);
            DbPath = Path.Join(path, "recept.db");
        }

        protected override void OnConfiguring(DbContextOptionsBuilder options) => options.UseSqlite($"Data source={DbPath}");

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Recipe>()
                .HasMany(r => r.Ingredient)
                .WithMany(r => r.Recipe)
                .UsingEntity<RecipeIngredient>(
                    j => j
                        .HasOne(ri => ri.Ingredient)
                        .WithMany(i => i.RecipeIngredients)
                        .HasForeignKey(ri => ri.IngredientId),
                    j => j
                        .HasOne(ri => ri.Recipe)
                        .WithMany(r => r.RecipeIngredients)
                        .HasForeignKey(ri => ri.RecipeId)
                );
        }

//Recipe, Ingredient and RecipeIngredient class
    public class Recipe
    {
        public int RecipeId { get; set; }
        public string Name { get; set; }

        public List<RecipeIngredient> RecipeIngredients { get; set; }
        public ICollection<Ingredient> Ingredient { get; set; }

    }

    public class Ingredient
    {
        public int IngredientId { get; set; }
        public string Name { get; set; }

        public List<RecipeIngredient> RecipeIngredients { get; set; }
        public ICollection<Recipe> Recipe { get; set; }

    }

    public class RecipeIngredient
    {
        public int IngredientId { get; set; }
        public Ingredient Ingredient { get; set; }
        public int RecipeId { get; set; }
        public Recipe Recipe { get; set; }
        public int Quantity { get; set; }
    }

編輯：從調試器添加結果：

Answer 1

解決了這個問題。

以錯誤的方式添加了 RecipeIngredients。

我將RecipeIngredients 添加到導致問題的Recipes 列表中。 它是通過參考配方和成分創建一個 Recipeingredient 對象，並將其添加到 DBset RecipeIngredient 來解決的。