EF Core - 如何包含子实体但限制返回的子实体数量

Question

I am using .NET Core 3.1. I have a parent with thousands of child elements, but I only want to retrieve the latest 10 children (and later will use pagination). When I run the following query:

context.Parent.Include(p => p.Child)

it will return all children and is incredibly slow. How can I return only 10 children and later use it for pagination? EF prevents using lambdas inside the Include, for example context.Parent.Include(p => p.Child.Take(10)) throws an exception.

Do I need multiple calls to the DB?

Answer 1

I hope this query helps.

var result = Parentlist
.Select(a => new { a, Childs = a.Childs.Take(10).ToList() })
.AsEnumerable() 
.Select(x =>
{
    x.a.Childs = x.Childs;
    return x.a;
}).ToList();

Answer 2

    var model = context.parent.Where(condition).OrderBy(x).Take(y).Skip(z)
    .select(c=> new parentViewModel 
    {
        Id = c.Id,
        Name= c.Name,
        children = c.children.where(condition)
                    .OrderByDescending(x=> x.CreateDate)
                    .take(pageSize)
                    .Select(x => new childViewModel {
                      Id = x.Id,
                      ChildName = x.Name
                       });
    }); 

Answer 3

What if you write the query starting from the children

Context.Chidlren.Where(x => condition ( x.Parent)).Include(x => x.Parent).Take(10).Select(x=> x.Parent)