public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }
    public List<Post> Posts { get; set; }
}

public class Post
{
    public int PostId { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }

    public int BlogId { get; set; }
    public Blog Blog { get; set; }
}

現(xiàn)在，我們需要向 Entity Framework Core 指明類型有一個附加列：

public class BloggingContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }
    public DbSet<Post> Posts { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<Post>()
            .Property<bool>("IsDeleted");
    }
}

更改插入、刪除查詢

Entity Framework Core 使用ChangeTracker存儲所有的更改。您可以在EF生成SQL語句和執(zhí)行這些語句之前修改ChangeTracker。

public class BloggingContext : DbContext
{
    public override int SaveChanges(bool acceptAllChangesOnSuccess)
    {
        OnBeforeSaving();
        return base.SaveChanges(acceptAllChangesOnSuccess);
    }

    public override Task<int> SaveChangesAsync(bool acceptAllChangesOnSuccess, CancellationToken cancellationToken = default(CancellationToken))
    {
        OnBeforeSaving();
        return base.SaveChangesAsync(acceptAllChangesOnSuccess, cancellationToken);
    }

    private void OnBeforeSaving()
    {
        foreach (var entry in ChangeTracker.Entries<Post>())
        {
            switch (entry.State)
            {
                case EntityState.Added:
                    entry.CurrentValues["IsDeleted"] = false;
                    break;

                case EntityState.Deleted:
                    entry.State = EntityState.Modified;
                    entry.CurrentValues["IsDeleted"] = true;
                    break;
            }
        }
    }
}

現(xiàn)在生成以下代碼執(zhí)行的SQL語句：

using (var context = new BloggingContext())
{
    var post = new Post { Blog = blog };
    context.Posts.Add(post);
    context.SaveChanges();
}

exec sp_executesql N'SET NOCOUNT ON;
INSERT INTO [Posts] ([BlogId], [Content], [IsDeleted], [Title])
VALUES (@p1, @p2, @p3, @p4);
SELECT [PostId]
FROM [Posts]
WHERE @@ROWCOUNT = 1 AND [PostId] = scope_identity();
-- @p3 is 0 (false)
',N'@p1 int,@p2 nvarchar(4000),@p3 bit,@p4 nvarchar(4000)',@p1=1,@p2=NULL,@p3=0,@p4=NULL

    context.Posts.Remove(post);
    context.SaveChanges();

exec sp_executesql N'SET NOCOUNT ON;
UPDATE [Posts] SET [BlogId] = @p0, [Content] = @p1, [IsDeleted] = @p2, [Title] = @p3
WHERE [PostId] = @p4;
SELECT @@ROWCOUNT;

',N'@p4 int,@p0 int,@p1 nvarchar(4000),@p2 bit,@p3 nvarchar(4000)',@p4=1,@p0=1,@p1=NULL,@p2=1,@p3=NULL

插入和刪除請求已經(jīng)被處理，您現(xiàn)在還必須更改所有查詢語句。

更改查詢語句

Entity Framework Core 2.0 引入了一個新的概念：查詢過濾器。查詢過濾器總是在生成的查詢語句后面追加一個的where子句。這意味著，您可以在模型創(chuàng)建時聲明一個實體的過濾器，然后將此過濾器隱式添加到使用該表的生成的每個查詢語句中。

public class BloggingContext : DbContext
{
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<Post>()
            .Property<bool>("IsDeleted");

        modelBuilder.Entity<Post>()
            .HasQueryFilter(post => EF.Property<bool>(post, "IsDeleted") == false);
    }
}

讓我們看看查詢過濾器的作用：

 var posts = context.Posts.ToList();

SELECT [p].[PostId], [p].[BlogId], [p].[Content], [p].[IsDeleted], [p].[Title]
FROM [Posts] AS [p]
WHERE [p].[IsDeleted] = 0 -- Query filter

查詢過濾器也可以用于關(guān)聯(lián)查詢：

 var blogs = context.Blogs.Include(_ => _.Posts);

SELECT [_].[BlogId], [_].[Url]
FROM [Blogs] AS [_]
ORDER BY [_].[BlogId]

SELECT [p].[PostId], [p].[BlogId], [p].[Content], [p].[IsDeleted], [p].[Title]
FROM [Posts] AS [p]
INNER JOIN (
    SELECT [_0].[BlogId]
    FROM [Blogs] AS [_0]
) AS [t] ON [p].[BlogId] = [t].[BlogId]
WHERE [p].[IsDeleted] = 0 -- Query filter
ORDER BY [t].[BlogId]

通過查詢過濾器實現(xiàn)軟刪除非常容易

查詢軟刪除的行

如果您要還原已刪除的行，您必須能夠查詢到這些數(shù)據(jù)。這意味著您需要臨時刪除查詢過濾器。EF已經(jīng)添加了一種新方法IgnoreQueryFilters來表明您不希望將查詢過濾器用于當前查詢。

var deletedPosts = context.Posts.IgnoreQueryFilters()
                    .Where(post => EF.Property<bool>(post, "IsDeleted") == true);

恢復(fù)已刪除的帖子有點啰嗦，您需要更改跟蹤器中查詢并更新IsDeleted屬性。

var deletedPosts = context.Posts.IgnoreQueryFilters().Where(post => EF.Property<bool>(post, "IsDeleted") == true);
foreach (var deletedPost in deletedPosts)
{
    var postEntry = context.ChangeTracker.Entries<Post>().First(entry => entry.Entity == deletedPost);
    postEntry.Property("IsDeleted").CurrentValue = false;
}
context.SaveChanges();