如何避免多个查询EF .net核心中的一个复杂对象

Question

In .net core 2.2 I have a model likes this:

public class A
{
  public ICollection<B> B { get; set; }
}

public class B 
{
  public ICollection<C> C { get; set; }
  public ICollection<D> D { get; set; }
}

public class C
{

}

public class D
{

}

And the query is something like this:

var query = from record in Set<A>()
            where record.Id == Id
select new AProjection
{
    BProjection =( from bRecord in Set<B>() 
                    where bRecord.Id = record.BId
    select new BProjection 
    {
        CProjection = bRecord.C.Select( b => new CProjection {/* data here */ }),
        DProjection = bRecord.C.Select( b => new DProjection {/* data here */ })
    })
}

I want to return the data into a projection, but for each object of Type B is making a query to get the objects Type C and another to get the objects Type D. How can I avoid this behaviour?

It's the typical N+1 queries problem addressed in this question:

How to avoid n+1 queries in EF Core 2.1?

Answer 1

EF Core 2.1 has introduced Optimization of correlated subqueries , but as mentioned in the linked documentation, you need to opt-in for it by adding ToList() to all collection projections.

Also use navigation properties instead of manual joins where possible.

eg

var query =
    from a in db.Set<A>()
    select new AProjection
    {
        B = (from b in a.B
             select new BProjection
             {
                 C = (from c in b.C select new CProjection { ... }).ToList(), // <--
                 D = (from d in b.D select new DProjection { ... }).ToList(), // <--
             }).ToList() // <--
    };

This will execute total 4 SQL queries - one for each projection.