如何在DOT NET ENTITY代码中进行组时，如何选择第一个组中每个组中的无效字段

Question

我有一个带有字段类型字符串的模型。

我想通过 get 第一个 group> group 的项目进行组中的查询。

但是，对于第一个项目字符串字段是而不是null

我在代码中运行此查询，但是出现错误，

var items = appDbContext.Foods.GroupBy(r => r.Category)
    .Select(g => new { g.Key,image= g.Where(r=>!string.IsNullOrEmpty(r.Image)).First().Image??"" })
    .ToDictionary(k => k.Key, k => k.image);

我的错误：

System.InvalidOperationException
  HResult=0x80131509
  Message=The LINQ expression 'GroupByShaperExpression:
KeySelector: f.Category, 
ElementSelector:EntityShaperExpression: 
    EntityType: Foods
    ValueBufferExpression: 
        ProjectionBindingExpression: EmptyProjectionMember
    IsNullable: False

    .Where(r => !(string.IsNullOrEmpty(r.Image)))
    .Select(s => s.Image)
    .First()' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

我的c＃模型 ：

    public class Foods
    {
        public int FoodId { get; set; }
        public string Name { get; set; }
        public int Price { get; set; }
        public string Image { get; set; }
        public string Category { get; set; }
        public string Description { get; set; }
        public DateTimeOffset InsertDate { get; set; }
        public bool IsActive { get; set; }
    }

Answer 1

假设您想在没有图像的情况下隐藏类别，则可以这样做：

foods.Where(x => !string.IsNullOrEmpty(x.Image))
    .GroupBy(x => x.Category)
    .Select(x => x.First());

基本上，您可以在没有图像的情况下滤除所有项目，然后使用组。但是，如果类别（组）没有图像的所有项目，则将跳过。

如果您也想拥有无效类别，这对我有用（请注意，这是通过内存收集进行测试的，而不是EF）

var foods = new List<Foods>()
{
    new Foods()
    {
        FoodId = 1,
        Name = "Test1",
        Price = 20,
        Image = "my-image",
        Category = "category1",
        Description = "description1",
        InsertDate = DateTimeOffset.Now,
        IsActive = true
    },
    new Foods()
    {
        FoodId = 1,
        Name = "Test2",
        Price = 20,
        Image = "my-image",
        Category = "category1",
        Description = "description1",
        InsertDate = DateTimeOffset.Now,
        IsActive = true
    },
    new Foods()
    {
        FoodId = 1,
        Name = "Test3",
        Price = 20,
        Category = "category2",
        Description = "description1",
        InsertDate = DateTimeOffset.Now,
        IsActive = true
    },
    new Foods()
    {
        FoodId = 1,
        Name = "Test4",
        Price = 20,
        Category = "category2",
        Description = "description1",
        InsertDate = DateTimeOffset.Now,
        IsActive = true
    }
};

var filteredFoods = foods
    .GroupBy(x => x.Category)
    .Select(x => x.FirstOrDefault( t=> !string.IsNullOrEmpty(t.Image)) ?? x.First());