LINQ to SQL eager loading with conditions

问题

I'm trying to learn LINQ to SQL and i've found out about the LoadWith function. All the examples i've found will load all records from the table you specify in the LoadWith function e.g.

var dlo = new DataLoadOptions();
dlo.LoadWith<Blog>(b => b.Posts);
this.LoadOptions = dlo;

What I would like to know is if it's possible to load in this example only the last blog post?

I've tried

dlo.LoadWith<Blog>(b => b.Posts.Max());

But it doesn't like that syntax.

回答1:

You can do it using AssociateWith. This will work:

var options = new DataLoadOptions();
options.AssociateWith<Blog>(b => 
    b.Posts.Where(
        p1 => p1.SomeColumn == b.Posts.Max(p2 => p2. SomeColumn)
    ));

Also, if you will be loading the info into a separate class or can use an anonymous one you can just do the query as:

var query = from b in context.Blogs
            //probably some where you already have
            select new MyBlogs // or with no type in case it is anonymous
            {
                AColumn = b.AColumn, //map any other values
                LatestPost = b.Posts.Where(
                      p1 => p1.SomeColumn == b.Posts.Max(p2 => p2. SomeColumn)
                  )).ToList()
            }

回答2:

If you only want the last post then I suspect that simply querying for that post specifically will be more efficient using lazy loading than forcing an 'eager' load this way.

来源：https://stackoverflow.com/questions/710029/linq-to-sql-eager-loading-with-conditions