如何在Linq中避免“选择n + 1”模式

Question

I have a query (including LinqKit) of the form:

Expression<Func<Country, DateTime, bool>> countryIndepBeforeExpr = 
  (ct, dt) => ct.IndependenceDate <= dt;
DateTime someDate = GetSomeDate();
var q = db.Continent.AsExpandable().Select(c =>
  new 
  {
    c.ID,
    c.Name,
    c.Area,
    Countries = c.Countries.AsQueryable()
                 .Where(ct => countryIndepBeforeExpr.Invoke(ct, someDate))
                 .Select(ct => new {ct.ID, ct.Name, ct.IndependenceDate})
  });

Now I want to iterate through q ... but since the Countries property of each element is of type IQueryable , it will be lazy loaded, causing n+1 queries to be executed, which isn't very nice.

What is the correct way to write this query so that all necessary data will be fetched in a single query to the db?

EDIT

Hm, well it might have helped if I had actually run a Sql trace before asking this question. I assumed that because the inner property was of type IQueryable that it would be lazy-loaded... but after doing some actual testing, it turns out that Linq to Entities is smart enough to run the whole query at once.

Sorry to waste all your time. I would delete the question, but since it already has answers, I can't. Maybe it can serve as some kind of warning to others to test your hypothesis before assuming it to be true!

Answer 1

Include countries to your model when you call for continents. With something like this:

var continents = db.Continent.Include(c => c.Countries).ToArray();

Then you can make your linq operations without iQueryable object.

Answer 2

I think this should work (moving AsExpandable() to root of IQueryable ):

var q = db.Continent
          .AsExpandable()
          .Select(c => new 
          {
              c.ID,
              c.Name,
              c.Area,
              Countries = c.Countries
                  .Where(ct => countryIndepBeforeExpr.Invoke(ct, someDate))
                  .Select(ct => new {ct.ID, ct.Name, ct.IndependenceDate})
});

If not, create two IQueryable and join them together:

var continents = db.Continents;
var countries = db.Countries
                  .AsExpandable()
                  .Where(c => countryIndepBeforeExpr.Invoke(c, someDate))
                  .Select(c => new { c.ID, c.Name, c.IndependenceDate });

var q = continents.GroupJoin(countries,
    continent => continent.ID,
    country => country.ContinentId,
    (continent, countries) => new
    {
        continent.ID,
        continent.Name,
        continent.Area,
        Countries = countries.Select(c => new
        {
            c.ID,
            c.Name,
            c.IndependenceDate
        })
    });