实体框架排除字段查询计数和POCO最佳方法

Question

I created a qr object to get total item count without getting all data from database and also if necessary to select MyViewModel list

var qr = dbSet.Select(o => 
                          new { ParentID = o.Parent.ID, o.Parent.Name, o.ID})
.Select(o => new MyViewModel(o.ParentID, o.Name, o.ID));

But when I tried qr.Count() and qr.ToList() they all run same query in database which is for items not only for items' count.

What is the best and fastest way to get 'MyViewModel' items' itself or items' count or both at the same time?

Answer 1

You have .AsEnumerable() call in your linq query. That means that any translate-your-query-into-SQL stops right there, and anything after will not be translated to SQL. Therefore, the only query that will be run in your database is the retrieval of you complete dbSet . If you want to only retrieve the count, than run the following linq query

var qrCount = dbSet.Count();

Answer 2

The problem with this query is because it calls a constructor of MyViewModel in

.Select(o => new MyViewModel(o.ParentID, o.Name, o.ID))

This call effectively converts your IQueryable to IEnumerable, as it is not possible to call a constructor from a query.

To avoid this conversion, use object initializer instead of constructor:

.Select(o => new MyViewModel 
        { 
            ParentID = o.ParentID, 
            Name = o.Name, 
            ID = o.ID
        })