使用 Linq 对 SETS 执行 CRUD：如何？

Question

使用 Linq，尝试选择不在本地集中的所有记录失败：

var localset = new List<Category>();
localset.Add( new Category { pk1 = 1, pk2 = 1, name = "one" } );
localset.Add( new Category { pk1 = 1, pk2 = 2, name = "two" } );

var dbCategories = dc.Categories;

var diff = dbCategories.Except(localset);

我需要执行基本的 CRUD：不在本地集中时从数据库集中删除，更新存在的位置，并在不存在时添加新记录。

我通常会这样做：

delete from Category C where not exists
    ( select null from LocalSet L where C.pk1 = L.pk1 and C.pk2 = L.pk2 )

update Category set name = L.name
from LocalSet L
where L.pk1 = Category.pk1 and L.pk2 = Category.pk2

insert into Category (pk1, pk2, name)
select pk1, pk2, name
from LocalSet L
where not exists (
    select null from Category C where L.pk1 = C.pk1 and L.pk2 = C.pk2 )

我想很简单。然而，.Contains 似乎是唯一适用于本地集的方法，并且它似乎只比较单个字段。数据库表具有复合键。

在不改变组合键的情况下，有什么办法可以完成这些任务呢？

Answer 1

您必须实现如下所示的 CategoryComparer：

public class CategoryComparer : IEqualityComparer<Category>
{
    public bool Equals(Category a, Category b)
    {
        bool result = false;
        if( a.pk1 == b.pk1 && a.pk2 == b.pk2 && a.name == b.name)
            result = true;
        return result;    
    }

    public int GetHashCode(Category category)
    {        
        if (Object.ReferenceEquals(category, null)) return 0;        
        return category.pk1.GetHashCode() ^ category.pk2.GetHashCode();
    }
}

然后调用 Enumerable.Except：

var localset = new List<Category>();
localset.Add( new Category { pk1 = 1, pk2 = 1, name = "one" } );
localset.Add( new Category { pk1 = 1, pk2 = 2, name = "two" } );

var diff = dc.Categories
             .AsEnumerable()
             .Except(localset, new CategoryComparer());

让我知道这是否能满足您的需要。

Answer 2

恐怕我不清楚您正在谈论哪种 Contains 方法......但是使用组合键的通用解决方案是使用匿名类型创建单个值。

例如，如果您想在复合键上加入，您可以使用

from x in xs
join y in ys on new { x.k1, x.k2 } equals new { y.k1, y.k2 }

Does that help?

Answer 3

这样做怎么样：

var localset = new List<Category>();
localset.Add(new Category { pk1 = 1, pk2 = 1, name = "one" });
localset.Add(new Category { pk1 = 1, pk2 = 2, name = "two" });

var flattened = localset
    .Select(dd => dd.pk1 + "|" + dd.pk2 + "|" + dd.name)
    .ToArray();

var dbCategories = dc.Categories;

var diff = from c in dbCategories
           let x = c.pk1 + "|" + c.pk2 + "|" + c.name
           where !flattened.Contains(x)
           select c;

这会将复合键转换为单个数组，允许您使用 Contains 运算符。如果有帮助请告诉我。干杯。