Entity Framework: Query is slow

Question

I am having some problems with Entity Framework. I have simplified this to make it easier to explain.

These are my mssql tables

I use the following code to get all cities for each of the countries in my MSSQL database

var country = new Country()
{
    Cities = obj.Counties.SelectMany(e => e.Cities).Select(city => new DCCity
    {
        Name = city.Name,
        Population = city.Population
    })
};

This is returned as json

There is a bit more then 40.000 records in the city table. To retrieve a list with all the countries and their respective cities it takes around 8 seconds. I am trying to reduce this. Anyone know some optimization tips to achieve this?

Answer 1

You need to query the Cities table first to get all data:

var cities = _context.Cities.Select(x => new {
    ContryId = x.County.Country.CountryId,
    ContryName = x.County.Country.Name,
    CityId = x.Id,
    CityName = x.Name
});

var countryLookup = new Dictionary<int, CountryDto>(approximatelyCountOfCountries);

foreach (var city in cities)
{
    CountryDto country;
    if (!countryLookup.TryGetValue(city.CountryId, out country))
    {
        country = new CountryDto {
            Name = city.CountryName,
            Id = city.CountryId
            Cities = new List<CityDto>(approximatelyCountOfCities)
        };
        countryLookup.Add(country.Id, country);
    }
    country.Cities.Add(new CityDto { Name = city.Name, Id = city.Id });
}

In this way the result will be the:

countryLookup.Values

Answer 2

Try to do somthing like this:

            var result = from c in countries
                     join conty in counties on c.id equals conty.CountryId
                     join city in cities on conty.id equals city.CountyId

                     group city by c.Name into g

                     select new
                     {
                         Name = g.Key,
                         Cities = g.Select(x =>
                             new
                             {
                                 x.Name,
                                 x.Population
                             })
                     };