Don’t Use LINQ’s Join. Navigate!

One of the greatest benefits of LINQ to SQL and LINQ to Entities is navigation properties that allows queries across several tables, without the need to use explicit joins. Unfortunately LINQ queries are often written as a direct translation of a SQL query, without taking advantage of the richer features offered by LINQ to SQL and LINQ to Entities.

It is not uncommon to see code doing a join manually.

from p in ctx.Persons
join c in ctx.Cities
on p.BornIn equals c.CityID
select new
{
    p.FirstName,
    c.Name
};

I think that using navigation properties makes the code much easier to read.

from p in ctx.Persons
select new
{
    p.FirstName,
    p.BornInCity.Name
};

Explicit Joins Taking Over

The first result for the google search for “linq-to-sql join” shows how to do several types of joins, but never mentions navigation properties.

On Stack Overflow there are plenty of questions about joins. The sad thing is that most of them are answered with help on doing explicit joins. Navigation properties are hardly ever mentioned.

A More Convincing Example

If there still is anyone in doubt of the benefits of navigation properties I’ve created another example.

from p in ctx.Persons
where p.ID == personId
join bornIn in ctx.Cities
on p.BornIn equals bornIn.CityID
join livesIn in ctx.Cities
on p.LivesIn equals livesIn.CityID
join s in ctx.Sexes
on p.SexID equals s.ID
select new PersonInfo
{
    Name = p.FirstName + " " + p.LastName,
    BornIn = bornIn.Name,
    LivesIn = livesIn.Name,
    Gender = s.Name,
    CarsOwnedCount = ctx.Cars.Where(c => c.OwnerID == p.ID).Count()
}

Especially note the double join against the City table. I very much prefer the query with navigation properties.

from p in ctx.Persons
where p.ID == personId
select new PersonInfo
{
    Name = p.FirstName + " " + p.LastName,
    BornIn = p.BornInCity.Name,
    LivesIn = p.LivesInCity.Name,
    Gender = p.Sex.Name,
    CarsOwnedCount = p.Cars.Count(),
}

It is worth noting that both these queries generate the same SQL when executed (to be honest, the left and right side of three comparisons are swapped, so the text is actually different). There is no performance impact of using navigation properties.

Navigation Properties and Foreign Keys

As long as there are proper foreign key constraints in the database, the navigation properties will be created automatically. It is also possible to manually add them in the ORM designer. As will all LINQ to SQL usage I think that it is best to focus on getting the database right and have the code exactly reflect the database structure. With the relations properly specified as foreign keys the code can safely make assumptions about referential integrity between the tables.

Please Promote Navigation Properties

Please help me promote navigation properties and discourage use of explicit joins unless really needed (yes, there are cases where navigation properties are not suitable, but they are a small minority). If you are on Stack Overflow answering questions, please point out whenever navigation properties can be used instead of just providing the join.