Linq 集团（按关联实体）

Question

我想获得他们列表中最多的项目 我要运行的 sql 查询

select 
    i.Id, 
    count(*) as count
from Items
    inner join ItemItemList il on i.Id = il.ItemsId
group by i.Id
order by count desc

物品实体

public  class Item:BaseEntity
{
    public string Name { get; set; }
    public decimal Price { get; set; }
    public decimal DiscountedPrice{ get; set; }
    public virtual ICollection<ItemList> ItemLists { get; set; }
}

项目列表实体

public class ItemList:BaseEntity
{
    public string Name { get; set; }
    public string Description { get; set; }
    public int UserId { get; set; }
    public  ICollection<Item> Items { get; set; }
    [ForeignKey("UserId")]
    public virtual User User { get; set; }
}

我的 DTO

public class TopItemsInLists
{
    [BsonRepresentation(BsonType.ObjectId)]
    [BsonId]
    public string ItemId { get; set; }
    public int Quantity { get; set; }
}

我的项目仓库

var query = _context.Items.Include(l => l.ItemLists)
            .GroupBy(g => g.ItemLists)
            .Select(z => new TopItemsInLists { ItemId = z.Key.ToString(), Quantity = z.Count() })
            .OrderByDescending(z => z.Quantity)
            .Take(10);

我想在 ItemLists 中找到最多的项目。 我在哪里做错了？如果有人有其他建议

Answer 1

您必须按 Id 分组，而不是按集合属性分组。就像在 SQL 中一样。但在此之前，您必须将记录乘以 SelectMany。

var query = _context.Items
    .SelectMany(i => i.ItemLists, (i, il) => i)
    .GroupBy(i => i.Id)
    .Select(g => new TopItemsInLists { ItemId = g.Key.ToString(), Quantity = g.Count() })
    .OrderByDescending(z => z.Quantity)
    .Take(10);

但我建议对这种更接近 SQL 的结构使用查询语法：

var query = 
    from i in _context.Items
    from il in i.ItemLists 
    group i by i.Id into g 
    select new TopItemsInLists
    {
        ItemId = g.Key.ToString(), 
        Quantity = g.Count()
    };

query = query
    .OrderByDescending(z => z.Quantity)
    .Take(10);