[Opf3] Question: Create one-to-many relations

Christian,

We are evaluating your product here at our company and have a question on WeakPersistent relationships. I have successfuly created a many-to-many WeakPersistent relationship such that removing a mapped object from the ObjectSet of the other does not delete it from the database. However, I am stuck on how to do the same for a one-to-many relationship. Such a relationship has no mapping table in the storage so I cannot create a matching class in the code. What am I missing here?

Thanx
Jay

Answer:
Hi Jay, you don't need any mapping class in a 1:n relationship. It is rather simple to create that one:

[Peristent("FOO")]
public class Foo
{
    private int _id;
    private string _name;

    // Create an ObjectSetHolder of Bar to connect with
    // Bar objects. The relation connects the property "ID"
    // of this class with the "FooID" property of the
    // related objects.
    [Relation("ID = FooID")]
    private ObjectSetHolder<Bar> _bars = new ObjectSetHolder<Bar>();

    public ObjectSet Bars
    {
       get { return _bars.InnerObject; }
    }

    [Field("ID", AllowDBNull = false, Identifier = true, AutoNumber = true)]
    public int ID
    {
        get { return _id; }
        private set { _id = value; }
    }

    [Field("NAME", AllowDBNull = false)]
    public string Name
    {
        get { return _name; }
        set { _name = value; }
    }
}

public class Bar
{
    private int _id;
    private string _text;
    private int _fooID;

    [Field("ID", AllowDBNull = false, Identifier = true, AutoNumber = true)]
    public int ID
    {
        get { return _id; }
        private set { _id = value; }
    }

    [Field("FOO_ID", AllowDBNull = false)]
    public string FooID
    {
        get { return _fooID; }
        set { _fooID = value; }
    }

    [Field("TEXT", AllowDBNull = false)]
    public string Text
    {
        get { return _text; }
        set { _text = value; }
    }
}

As you see you have only to create (in the class that should then contain the list of related objects) an ObjectSetHolder and decorate that class with a RelationAttribute. The first argument of the RelationAttribute's constructor is the property of the single object in the 1:n relation. The second argument is the property of the n objects (the property that is mapped to the foreign key in the database) in the 1:n relation.

You could have a look at the "Relations" sample that is part of the "tutorial and samples" from the Opf3 homepage. That sample contains a one-to-many relation.

If you delete a related object in a one-to-many or one-to-one relation that related object is deleted when you save the "parent" object. Example:

// Load a foo object from the storage.
Foo foo = context.GetObject("ID = 1");
// Delete the first object.
foo.Bars.RemoveAt(0);

// Save the instance of Foo.
// The removed instance of Bar is deleted form the storage.
// In a one-to-many and one-to-one relation related objects
// are deleted from the storage if they are removed.
context.PersistChanges(foo);

In a many-to-many relation the related object isn't deleted because because it may be still connected with another item in the database.

I hope I could help you with this post. If you (or anybody else) has any further question don't hesitate to post them here as comments