代码之家 › 专栏 › 技术社区 › Kacper

对连接表的引用引发ArgumentNullException“值不能为null”

entity-framework-core .net-core linq entity-framework c#

Kacper · 技术社区 · 7 年前

我正试着通过连接表为我的食谱找到配料。

_context.RecipeIngredients
    .Include(rI => rI.Recipe)
        .ThenInclude(r => r.RecipeIngredients)
    .Where(rI => ingredients.Contains(rI.IngredientId))
    .GroupBy(rI => rI.Recipe)
    .Select(g => new
    {
        Recipe = g.Key,
        MatchingIngredients = (double)g.Count() / (double)g.Key.RecipeIngredients.Count(),
        g.Key.ComplexityTag,
        g.Key.TypeTag,
        g.Key.RecipeIngredients,
    })
    .OrderByDescending(r => r.MatchingIngredients)
    .Take(MaxAmountBestRecipes)
    .AsEnumerable()
    .Select(a => new RecipeDTO()
    {
        Title = a.Recipe.Title,
        Steps = a.Recipe.Steps,
        ComplexityTag = a.ComplexityTag?.Complexity,
        TypeTag = a.TypeTag?.Type,
        IngredientAmount = a.RecipeIngredients?.ToDictionary(rI => rI.Ingredient.Name, rI => rI.Quantity),
    })
    .ToList();

我发现这是由g.Key引起的。收到的消息,但我找不到解决这个问题的方法。我尝试了eagar加载(如您所见)和惰性加载,但都不起作用。我希望在linq中对db的一次查询中能找到解决方案。此外,在更新后,它是否会像上面这行一样工作:

IngredientAmount = a.RecipeIngredients?.ToDictionary(rI => rI.Ingredient.Name, rI => rI.Quantity)

编辑我已经划分了linq查询,下面是在哪个语句之后抛出ArgumentNullException:

var tmp = _context.RecipeIngredients
            .Where(rI => ingredients.Contains(rI.IngredientId))
            .GroupBy(rI => rI.Recipe)
            .Select(g => new
            {
                Recipe = g.Key,
                MatchingIngredients = (double)g.Count() / (double)g.Key.RecipeIngredients.Count(),
                g.Key.ComplexityTag,
                g.Key.TypeTag,
                g.Key.RecipeIngredients
            })
            .ToList();

1 回复 | 直到 7 年前

Moho 7 年前

Include 当您更改查询的形状时(例如,当您查询 RecipeIngredient 但它们正投射到另一种类型)。

我认为空值唯一的问题是创建字典时的键选择器。自从 包括 不会为你做任何事, ri.Ingredient 将始终为空。包括 Ingredient s在原始投影中(并移除 包括 既然没用):

_context.RecipeIngredients
    .Where( rI => ingredients.Contains( rI.IngredientId ) )
    .GroupBy( rI => rI.Recipe )
    .Select( g => new
    {
        Recipe = g.Key,
        MatchingIngredientCount = (double)g.Count() / (double)g.Key.RecipeIngredients.Count(),
        g.Key.ComplexityTag,
        g.Key.TypeTag,
        g.Key.RecipeIngredients,
        // this eager-loads the `Ingredient` entities
        //   EF will automatically wire them up to the `RecipeIngredient` entities
        //   if tracking is enabled
        Ingredients = g.Key.RecipeIngredients.Select( ri => ri.Ingredient ),
    } )
    .OrderByDescending(r => r.MatchingIngredients)
    .Take(MaxAmountBestRecipes)
    .ToArray()
    .Select( a = new ...
    {
        ...
        IngredientAmount = a.RecipeIngredients.ToDictionary(
            ri => ri.Ingredient.Name, // ri.Ingredient should now not be NULL
            ri => ri.Quantity )
    } );

编辑:如果你不需要整个 接受方 或 Recipe 在结果中的实体,只需使用 接受方 在第一个投影中:

IngredientNamesAndQuantity = g.Key.RecipeIngredients.Select( ri => new
{
    ri.Quantity,
    ri.Ingredient.Name,
}

然后使用该投影构建词典:

IngredientAmount = a.IngredientNamesAndQuantity.ToDictionary(
    at => at.Name,
    at => at.Quantity )