How do I update multiple Entity models in one SQL statement?

问题

I had the following:

List<Message> unreadMessages = this.context.Messages
            .Where( x =>
                x.AncestorMessage.MessageID == ancestorMessageID &&
                x.Read == false &&
                x.SentTo.Id == userID ).ToList();

foreach(var unreadMessage in unreadMessages)
{
    unreadMessage.Read = true;
}

this.context.SaveChanges();

But there must be a way of doing this without having to do 2 SQL queries, one for selecting the items, and one for updating the list.

How do i do this?

回答1:

Current idiomatic support in EF

As far as I know, there is no direct support for "bulk updates" yet in Entity Framework (there has been an ongoing discussion for bulk operation support for a while though, and it is likely it will be included at some point).

(Why) Do you want to do this?

It is clear that this is an operation that, in native SQL, can be achieved in a single statement, and provides some significant advantages over the approach followed in your question. Using the single SQL statement, only a very small amount of I/O is required between client and DB server, and the statement itself can be completely executed and optimized by the DB server. No need to transfer to and iterate through a potentially large result set client side, just to update one or two fields and send this back the other way.

How

So although not directly supported by EF, it is still possible to do this, using one of two approaches.

Option A. Handcode your SQL update statement

This is a very simple approach, that does not require any other tools/packages and can be performed Async as well:

var sql = "UPDATE TABLE x SET FIELDA = @fieldA WHERE FIELDB = @fieldb";
var parameters = new SqlParameter[] { ..., ... };
int result = db.Database.ExecuteSqlCommand(sql, parameters);

int result = await db.Database.ExecuteSqlCommandAsync(sql, parameters);

The obvious downside is, well breaking the nice linqy paradigm and having to handcode your SQL (possibly for more than one target SQL dialect).

Option B. Use one of the EF extension/utility packages

Since a while, a number of open source nuget packages are available that offer specific extensions to EF. A number of them do provide a nice "linqy" way to issue a single update SQL statement to the server. Two examples are:

Entity Framework Extended Library that allows performing a bulk update using a statement like:
```
context.Messages.Update(
     x => x.Read == false && x.SentTo.Id == userID,
     x => new Message { Read = true });
```
It is also available on github

EntityFramework.Utilities that allows performing a bulk update using a statement like:

EFBatchOperation
   .For(context, context.Messages)
   .Where(x => x.Read == false && x.SentTo.Id == userID)
   .Update(x => x.Read, x => x.Read = true);

It is also available on github

And there are definitely other packages and libraries out there that provide similar support.

回答2:

Even SQL has to do this in two steps in a sense, in that an UPDATE query with a WHERE clause first runs the equivalent of a SELECT behind the scenes, filtering via the WHERE clause, then applying the update. So really, I don't think you need to be worried about improving this.

Further, the reason why it's broken into two steps like this in LINQ is precisely for performance reasons. You want that "select" to be as minimal as possible, i.e. you don't want to load any more objects from the database into in memory objects than you have to. Only then do you alter objects (in the foreach).

If you really want to run a native UPDATE on the SQL side, you could use a System.Data.SqlClient.SqlCommand to issue the update, instead of having LINQ give you back objects that you then update. That will be faster, but then you conceptually move some of your logic out of your C# code object model space into the database model space (you are doing things in the database, not in your object space), even if the SqlCommand is being issued from your code.

来源：https://stackoverflow.com/questions/29551323/how-do-i-update-multiple-entity-models-in-one-sql-statement

标签

sql

entity-framework

sql-update

entity