Association with CanBeNull = false produces LeftJoin when using QueryExpressionMethod

LinqToDb-Version: 3.1.0 DB: Firebird 2.1

Hi,

I want to generate an inner join by using the association attribute and a QueryExpressionMethod.

Let’s say I have this structure:

public class Card{
    public int Id { get; set;}
    public string CardName { get; set;}
    [Association(QueryExpression = nameof(Card_Owner), CanBeNull = false)]
    public Client Owner {get; set; }
    public int OwnerId {get; set; }

    public static Expression<Func<Card, IDataContext, IQueryable<Client>>> Card_Owner()
    {
       return (c, db) => db.GetTable<Client>().Where(cl=> cl.Id == c.OwnerId);
    }
}

public class Client{
    public int Id { get; set;}
    public string Name {get; set; }
}

So if I call it :

using(var db = new MyDb()){
    var cards = db.GetTable<Card>().LoadWith(x => x.Client).ToList();
)

The resulting SQL produces a left join.

if I change the Association Attribute to :

public class Card{
    public int Id { get; set;}
    public string CardName { get; set;}
    [Association(ThisKey = "OwnerId", OtherKey = "Id", CanBeNull = false)]
    public Client Owner {get; set; }
    public int OwnerId {get; set; }
}

it produces an inner join. Although both have the CanBeNull set to false!

[sdanyliv]

Yes, in this case we ignore CanBeNull attribute. Will check what we can do here. But you have to understand if we use INNER JOIN, then association may filter out principal entities.

[travbeamo]

It might not be such an issue if the entities models correctly use Nullable Reference Types for association properties.

If the helper methods & code refactoring proposed in issue https://github.com/linq2db/linq2db/issues/2592 are accepted in to Linq2DB, then the canBeNull attribute could possibly be removed altogether.

Then the correct joins would be used purely based on the if the association properties in the entities model are nullable:

• If association property is nullable = LEFT join is used
• If association property is not nullable = INNER join is used