Entity framework linq query Include() multiple chi

Question

This may be a really elementry question but whats a nice way to include multiple children entities when writing a query that spans THREE levels (or more)?

i.e. I have 4 tables: Company, Employee, Employee_Car and Employee_Country

Company has a 1:m relationship with Employee.

Employee has a 1:m relationship with both Employee_Car and Employee_Country.

If i want to write a query that returns the data from all 4 the tables, I am currently writing:

Company company = context.Companies
                         .Include("Employee.Employee_Car")
                         .Include("Employee.Employee_Country")
                         .FirstOrDefault(c => c.Id == companyID);

There has to be a more elegant way! This is long winded and generates horrendous SQL

I am using EF4 with VS 2010

Answer 1

Use extension methods. Replace NameOfContext with the name of your object context.

public static class Extensions{
   public static IQueryable<Company> CompleteCompanies(this NameOfContext context){
         return context.Companies
             .Include("Employee.Employee_Car")
             .Include("Employee.Employee_Country") ;
     }

     public static Company CompanyById(this NameOfContext context, int companyID){
         return context.Companies
             .Include("Employee.Employee_Car")
             .Include("Employee.Employee_Country")
             .FirstOrDefault(c => c.Id == companyID) ;
      }

}

Then your code becomes

     Company company = 
          context.CompleteCompanies().FirstOrDefault(c => c.Id == companyID);

     //or if you want even more
     Company company = 
          context.CompanyById(companyID);

Answer 2

EF 4.1 to EF 6

There is a strongly typed .Include which allows the required depth of eager loading to be specified by providing Select expressions to the appropriate depth:

using System.Data.Entity; // NB!

var company = context.Companies
                     .Include(co => co.Employees.Select(emp => emp.Employee_Car))
                     .Include(co => co.Employees.Select(emp => emp.Employee_Country))
                     .FirstOrDefault(co => co.companyID == companyID);

The Sql generated in both instances is still by no means intuitive, but seems performant enough. I've put a small example on GitHub here

EF Core

EF Core has a new extension method, .ThenInclude(), although the syntax is slightly different:

var company = context.Companies
                     .Include(co => co.Employees)
                           .ThenInclude(emp => emp.Employee_Car)
                      ...

As per the docs, I would keep the extra 'indent' in the .ThenInclude to preserve your sanity.

Obsolete Info (Don't do this):

The multiple grandchildren loading could be done in one step, but this requires a rather awkward reversal back up the graph before heading down the next node (NB: This does NOT work with AsNoTracking() - you'll get a runtime error):

var company = context.Companies
         .Include(co => 
             co.Employees
                .Select(emp => emp.Employee_Car
                    .Select(ec => ec.Employee)
                    .Select(emp2 => emp2.Employee_Country)))
         .FirstOrDefault(co => co.companyID == companyID);

So I would stay with the first option (one Include per leaf entity depth model).

Answer 3

You might find this article of interest which is available at codeplex.com.

• Improving Entity Framework Query Performance Using Graph-Based Querying.

The article presents a new way of expressing queries that span multiple tables in the form of declarative graph shapes.

Moreover, the article contains a thorough performance comparison of this new approach with EF queries. This analysis shows that GBQ quickly outperforms EF queries.

Answer 4

How do you construct a LINQ to Entities query to load child objects directly, instead of calling a Reference property or Load()

There is no other way - except implementing lazy loading.

Or manual loading....

myobj = context.MyObjects.First();
myobj.ChildA.Load();
myobj.ChildB.Load();
...