Question

I have Class which worked perfectly with only the ID column as Key. Now I added the Name column as a Second Key (Using the Key(0) and Key(1) Attributes). But now I'm getting the following error message on Domain.Build: Default generator can serve hierarchy with exactly one key field.

<HierarchyRoot()> _
<TableMapping("Projects")> _
Public Class Project
    Inherits Entity

    Sub New()
        MyBase.New()
    End Sub

    <Key(0), Field()> _
    Public Property ID() As Integer
        Get
            Return GetFieldValue(Of Integer)("ID")
        End Get
        Private Set(ByVal value As Integer)
            SetFieldValue(Of Integer)("ID", value)
        End Set
    End Property

    <Field(Length:=128), Key(1)> _
    Public Property Name() As String
        Get
            Return GetFieldValue(Of String)("Name")
        End Get
        Set(ByVal value As String)
            SetFieldValue(Of String)("Name", value)
        End Set
    End Property

    <Field(Length:=256, Nullable:=True)> _
    Public Property Description() As String
        Get
            Return GetFieldValue(Of String)("Description")
        End Get
        Set(ByVal value As String)
            SetFieldValue(Of String)("Description", value)
        End Set
    End Property

    <Field()> _
    Public Property Creator() As Guid
        Get
            Return GetFieldValue(Of Guid)("Creator")
        End Get
        Set(ByVal value As Guid)
            SetFieldValue(Of Guid)("Creator", value)
        End Set
    End Property

End Class

Accepted Answer

The error indicates the following:

You don't implement your own constructors for your entity. This means a default one will be provided by VB.NET or C# compiler.
Default constructor (with empty argument list) always calls default constructor of base type (Entity in this case).
Default Entity constructor assigns a key to Entity using default key generator.
In this case default key generator can't provide a key for you: it is capable of providing primitive keys only.

So what could be done in your case to resolve the issue?

Option 1: manual key assignment

Add the following constructor to your class (sorry, C# notation):

public Project(int id, string name)
  : base(id, name) // That's one of ways to create an entity with pre-defined key
{
  // Key is already assigned at this point
  Debug.WriteLine("Project.Key = {0}", Key);
}

Option 2: semi-automatic key assignment

As far as I can judge, you actually need to automatically generate a part of composite key - i.e. ID field value must be auto-generated, but Name field value must be manually assigned.

The simplest way to achieve this is to:

a) Declare a dummy entity type having the same primitive key type as the one you'd like to generate:

[HierarchyRoot]
public class IntKeyedEntity : Entity {
  [Key, Field]
  public int Id { get; private set; }
}

This is necessary to ensure that key generator providing key values for its Id field (in this case, a default key generator for keys of Int32 type) will be available when Domain is built, i.e. necessary data structures will be created, and runtime KeyGenerator instance is available.

b) Add the following constructor to your class:

public Project(string name)
  : base(Key.Create<IntKeyedEntity>().Value.GetValueOrDefault(0), name)
  // We use the same constructor of `Entity` type, but generate
  // a part of the key relying on key generator of another type.
{
  // Key is already assigned at this point
  Debug.WriteLine("Project.Key = {0}", Key);
}

That's it.

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