Left Join in Entity Framework

Posted by Joe Enos on January 07, 2021 · 8 mins read

If you need a left join in Entity Framework, you have a couple options. First, if you’re using a real foreign key that just happens to be nullable, then you can use the regular navigation properties. But if you’re doing a left join manually, or with other factors, then you need to do things just a little differently:

Suppose we have the following database:

create table dbo.Foods (
FoodID int not null identity primary key
,FoodName varchar(100) not null
);
go
insert dbo.Foods (FoodName) values ('Pizza'), ('Chicken'), ('Potatoes'), ('Broccoli');
go
create table dbo.People (
PersonID int not null identity primary key
,FirstName varchar(100) not null
,FavoriteFoodID int null
,constraint FK_Person_FavoriteFoodID
foreign key (FavoriteFoodID) references dbo.Foods (FoodID)
);
go
insert dbo.People (FirstName, FavoriteFoodID)
values ('John', 1), ('Mary', 2), ('Pat', null);
go

We can build our Entity Framework tables as follows:

[Table("Foods")]
public class Food
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int FoodID { get; set; }
public string FoodName { get; set; } = default!;
}
[Table("People")]
public class Person
{
[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int PersonID { get; set; }
public string FirstName { get; set; } = default!;
public int? FavoriteFoodID { get; set; }

[ForeignKey(nameof(FavoriteFoodID))]
public Food? FavoriteFood { get; set; }
}
public class MyContext : DbContext
{
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
=> optionsBuilder.UseSqlServer(@"server=(localdb)\MSSQLLocalDB;database=sandbox;integrated security=true;");

public DbSet<Food> Foods { get; set; } = default!;
public DbSet<Person> People { get; set; } = default!;
}

The navigation property FavoriteFood gives us the ability to harness Entity Framework’s intelligence to build a query:

var firstQuery = (from p in context.People
select new
{
p.PersonID,
p.FirstName,
FoodID = (int?)p.FavoriteFood!.FoodID,
FoodName = (string?)p.FavoriteFood!.FoodName
});

But if that navigation property wasn’t there, then we have an alternative way of doing a left join:

var secondQuery = (from p in context.People
from f in context.Foods.Where(f => f.FoodID == p.FavoriteFoodID).DefaultIfEmpty()
select new
{
p.PersonID,
p.FirstName,
f.FoodID,
f.FoodName
}).ToArray();

This generates identical SQL to the first one. Note the DefaultIfEmpty call.

There are other ways, but I find this to be very easy to read and understand.

View code on GitHub