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I have created an base class which my entities inherit from that gives a property called IsNew which returns true if the the value of ID is 0; all of my tables use ID for the identifier.

 

public abstract class EntityBase
{
    public virtual int ID { get; set; }

    public bool IsNew
    {
        get
        {
            return ID == 0;
        }
    }
}

For this to work each of my entities need to use the override modifier for the ID property.  Using the O/R Designer it is possible to set the inheritance modifier for each property, but this is not possible using SQLMetal and I need to use SQLMetal to set the base class.

To overcome this I have a batch file that first creates my .dbml file using SQLMetal:

SQLMetal.exe /server:localhost /database:University /dbml:University.dbml /namespace:Entities /context:UniversityDataContext /pluralize

I then run a console application to find column nodes with the name of ID, and add the Modifier attribute:

EntityProcessor.exe University.dbml

The code for EntityProcessor looks like this:

 

static void Main(string[] args)
{
    try
    {
        //Ensure there is an argument for the DBML file
        if (args.Count() == 0)
        {
            throw new ApplicationException("DBML path expected.");
        }

        string dbml = args[0];

        //Load the DBML file
        XmlDocument xmlDoc = new XmlDocument();
        xmlDoc.Load(dbml);

        //Loop through the tables
        foreach (XmlNode node in xmlDoc.GetElementsByTagName("Table"))
        {
            //Loop through the nodes for the type
            foreach (XmlNode child in node.FirstChild.ChildNodes)
            {
                //Find the ID column node
                if (child.Name.Equals("Column") && child.Attributes["Name"].Value.Equals("ID"))
                {
                    //Create the modifier attribute to add to ID column
                    XmlAttribute modifierAttribute = xmlDoc.CreateAttribute("Modifier");
                    modifierAttribute.Value = "Override";
                    child.Attributes.Append(modifierAttribute);
                }
            }
        }

        //Save the updated DBML file
        xmlDoc.Save(dbml);

        Console.WriteLine("Processing complete");
    }
    catch (Exception ex)
    {
        Console.WriteLine("An error occured: {0}", ex.Message);
    }
}

Finally I use SQLMetal again to create my entities from the updated .dbml file:

SQLMetal.exe /code:University.cs /entitybase:EntityBase /namespace:Entities University.dbml

My new entities now have an ID property with an overrides modifier that looks like this:

[Column(Storage="_ID", AutoSync=AutoSync.OnInsert, DbType="Int NOT NULL IDENTITY", IsPrimaryKey=true, IsDbGenerated=true)]
public override int ID
{
    get
    {
        return this._ID;
    }
    set
    {
        if ((this._ID != value))
        {
            this.OnIDChanging(value);
            this.SendPropertyChanging();
            this._ID = value;
            this.SendPropertyChanged("ID");
            this.OnIDChanged();
        }
    }
}

I can now call IsNew on any of my entities to check if they have been inserted into the database.