Support Forum

Forum Home

NDO-Forum postings in english please, if possible.

Developing with NDO

Mapping primary...

Mapping primary keys

12/20/2006 12:22:46 PM

jesicca

2 posts

www.prosoft.org

Mapping primary keys
Modified By Mirko on 12/27/2006 7:49:51 PM)

I have business object classes which are all derived from one base class. The base class provides the ID field for the derived classes. How should I map the id columns of the derived classes, so i can access them? If i do the mapping like this I get the following runtime exception: "Can't find field boGender._nId.". In my application it's necessary to read the primary keys from the database. Here is my base class:

[NDOPersistent]
public class boStandard
{
    public int _nId;
    public int Id
    {
        get { return this._nId; }
        set { this._nId = value; }
    }
...
}

and a derived class:
[NDOPersistent]
public class boGender : boStandard
{
    private string _sName;
    private string _sSalutation;
    public string Name
    {
        get { return this._sName; }
        set { this._sName = value; }
    }
    public string Salutation
    {
        get { return this._sSalutation; }
        set { this._sSalutation = value; }
    }
}

Here is the NDO mapping:
I'm sure there must be a simple solution for my problem, but I can't find it somewhere in the documentation or support-forum

12/27/2006 6:42:29 PM

Mirko

109 posts

Re: Mapping primary keys

Hi Jesicca,

sorry for the late answer, the portal software has eaten up my first reply somehow.

What do you mean, if you write, you need to read the primary keys from the database? If you mean, you have autonumbered IDs there, you don't have to bother with it anyway, because that's the default behavior of NDO. If you want to read the value of an Id, use the following code:

Employee emp = new Employee();
PersistenceManager pm = new PersistenceManager;
pm.MakePersistent(emp);
pm.Save();

Console.WriteLine(emp.NDOObjectId.Id.Value);

In the most cases you don't need to map a field to the Id.

Note, that autonumbered IDs have some disadvantages compared with client-side generated keys. We recommend using Guid Ids. You can force NDO to use Guid Ids with the custom attribute [NDOOidType(typeof(Guid))] which may be applied to classes and to whole assemblies. NDO generates automatically a new ID before saving a new object.

I hope, this answers your question. If you have more questions, I'm happy to answer it.

Best Regards
Mirko

1/9/2007 8:52:44 AM

jesicca

2 posts

www.prosoft.org

Re: Mapping primary keys

Hi Mirko, thanks for your answer. The reason why I have to read the (autonumbered) primary keys is that I want browse objects in a form. If I navigate to the next, previous, etc. object, I need the Id of the current object to select the next object. Another possibility would be to load the complete list of objects from the database and iterate it. But I worry about performance because the list could get very long, including complex objects (so even lazy loading wouldn't help much). Does NDOObjectId.Id always read the Id from the database or do I have to save the object first? Do I have to define a property as ObjectId (e. g. by setting an attribute)? Thanks a lot for your help. Best regards, Jesicca

1/15/2007 7:05:09 PM

Mirko

109 posts

Re: Mapping primary keys

Hi Jesicca,

after the call to MakePersistent your object is still in a transient state, having a temporary id. NDO assings negative IDs for temporary IDs. While you save the object, the database generates the id and this new id will be read back by NDO. After saving the id is available via the NDOObjectId.Id.Value property.

But I think, browsing through Id's is not, what you want. What you might consider is writing a class which contains kind of a preview of your more complex objects. You query the preview objects first and if you need the real object you can construct an ObjectId using the ObjectId of the preview object and the type of the real object class. Our tutorial application shows, how it works. In this app you have the class PictureHeader for showing some Header information in a Grid or whatever UI to select your objects. Using the ObjectId of the PictureHeader a hollow Picture object is constructed, which will be filled from the database, if one of the persistent fields is touched.

Again: you don't have to assign a specific property or field as object id. NDO does it automatically. After compiling with the NDO enhancer enabled your classes magically implement an additional interface IPertsistentCapable which contains the NDOObjectId property.

I hope this answers your question. If not, just ask back.

Mirko

Page 1 of 1

NDO-Forum postings in english please, if possible.

Developing with NDO

Mapping primary...