Question

使用Linq，尝试选择不在我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);

我需要做基本的CRUD：当不在我的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的。

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 )

我想的很容易。但是，.Contains似乎是唯一适用于本地集的方法，它似乎只比较单个字段。数据库表有复合键。

如果不更改复合键，有没有办法完成这些任务？

Answer 1

我不清楚你正在谈论哪种Contains方法，我担心......但使用复合键的通用解决方案是使用匿名类型创建单个值。

例如，如果要在复合键上加入，可以使用

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

这有帮助吗？

Answer 2

这样做：

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允许您使用Contains运算符的单个数组。如果有帮助，请告诉我。欢呼声。

Answer 3

您必须实施以下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());

请告诉我这是否能满足您的需求。

使用Linq在SETS上执行CRUD：如何？

3 个答案: