LINQ查询，n + 1个问题

Question

I have a query with three sub queries and my problem is that the sub queries are run for each Country (n+1).

I have boiled down my query to make it easier to read, since the main query is around 70 rows, and changed the domain to make it more understandable.

I've tried including Cities/Mountains/Rivers and running .ToList() on the sub queries, but to no avail.

// The CountryDto class I'm selecting to.
public class CountryDto
{
    public string CountryName { get; set; }
    public IEnumerable<CityDto> CityDtos { get; set; }
    public IEnumerable<MountainDto> MountainDtos { get; set; }
    public IEnumerable<RiverDto> RiverDtos { get; set; }
}
// The query
var query = _db.Countries
    .Select(country => new CountryDto
    {
        CountryName = country.Name,
        CityDtos = country.Citites
            .Where(city => city.Population > 10000)
            .Select(city => new CityDto
            {
                Name = city.Name,
            }),
        MountainDtos = country.Mountains
            .Where(mountain => mountain.Height > 100)
            .Select(mountain => new MountainDto
            {
                Name = mountain.Name,
            }),
        RiverDtos = country.Rivers
            .Where(river => river.Length > 1000)
            .Select(river => new RiverDto
            {
                Name = river.Name,
            }),
    })
    .Where(c => c.CityDtos.Any() || c.MountainDtos.Any() || c.RiverDtos.Any());

var totalCount = query.Count();
var countries = await query.ToListAsync();

Answer 1

Entity Framework Core supports parts of the query being evaluated on the client and parts of it being pushed to the database. It is up to the database provider to determine which parts of the query will be evaluated in the database.

In your case, all .Any parts are evaluated in the client side I guess. You can configure your code to throw an exception for client evaluation.

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder
        .UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFQuerying;Trusted_Connection=True;")
        .ConfigureWarnings(warnings => warnings.Throw(RelationalEventId.QueryClientEvaluationWarning));
}

For more info https://docs.microsoft.com/en-us/ef/core/querying/client-eval

Answer 2

Ended up splitting the query into a .Where() part for the count and a .Select() part for the result, which removed my n+1 issue.

var query = await _db.Countries
    .Include(c => c.Cities)
    .Include(c => c.Mountains)
    .Include(c => c.Rivers)
    .Where(c => c.Cities.Any(city => city.Population > 10000)
        || c.Mountains.Any(mountain => mountain.Heigh > 1000)
        || c.River.Any(river => river.Length > 100000))
    .Where(c => c.Cities.Any() || c.Mountains.Any() || c.Rivers.Any())
    .ToListAsync();

var totalCount = query.Count();

var countries = query
    .Select(country => new CountryDto
    {
        CountryName = country.Name,
        CityDtos = country.Citites
            .Select(city => new CityDto
            {
                Name = city.Name,
            }),
        MountainDtos = country.Mountains
            .Select(mountain => new MountainDto
            {
                Name = mountain.Name,
            }),
        RiverDtos = country.Rivers
            .Select(river => new RiverDto
            {
                Name = river.Name,
            }),
    })
    .ToList();