使用实体框架读取复杂/相关数据

Question

I want to read (using EF) City and Country information like CityID, CityName, CountryID, CountryName from the tables City and Country, which has appropriate FK relation.

For this, I can use

1. from x in db.Cities.Include("Country") select x - This results in complex query which tries to read all unwanted columns

2. from x in db.Cities join y in db.Countries on x.CountryID equals y.CountryID select new CityDTO{ x.CityID, x.CityName, y.CountryID, y.CountryName} - This results in creating lots of DTO classes and transformations.

3. from x in db.CityView select x - this results in creating many views and whenever I want to get additional column, I need to update the edmx.

4. Create additional property called CountryName in City class

and use

from x in db.Cities select x
foreach (var city in x)
  city.CountryName = (from y  in db.Countries where ...)

*** This is because EF is not allowing me to create City object inside the query otherwise I would have used from x in db.Cities join y in db.Countries select new City{x.CityID, x.CountryID, x.CityName, y.CountryName}

What is the best way to go in this case (with less entities and performing query)?

Answer 1

How about

from x in db.Cities.Include("Country") 
select new { x.CityID, x.CityName, x.Country.CountryID, x.Country.CountryName }

This selects just the specified fields into an anon type, allowing you to select only cols you care about

To use an existing entity you can use a similar syntax, eg.

from x in db.Cities.Include("Country") 
select new City { 
         CityID = x.CityID, 
         CityName = x.CityName, 
         Country = new Country{ 
                       CountryID = x.Country.CountryID, 
                       CountryName = x.Country.CountryName 
                   } 
         }

If you look at the generated SQL for either of these expressions you will see that its only selecting the mentioned columns.

Do be aware however that the entity returned from both of these queries is not tracked, so you cant use it directly to perform an update.