Invalid variant type conversion

[Albert kobina Arhin]

Hi Linas, I am having issues mapping a MS SQL VarBinary Field to a String in a test App. Below is my PODO.

type
  [Entity]
  [Table('META_USERS')]
   TUser = class
     private
        [Column('USERID', [cpRequired, cpPrimaryKey, cpNotNull, cpDontInsert])]
        [AutoGenerated]
        FUserID : Int32;
        [Column('LOGIN_NAME', [])]
        FLoginName : string;
        [Column('FULL_NAME', [])]
        FFullName : string;
        [Column('PASSWORD', [])]
        FPassword : string;
     public
         property UserID : Int32 read FUserID write FUserID;
         property LoginName : string read FLoginName write FLoginName;
         property FullName : string read FFullName write FFullname ;
         property Password : string read FPassword write FPassword;
   end;

A DAO

 TUserDBO = class
    FDBSession : TSession;
    public
      constructor Create;
      destructor destroy;overload;
      function GetAllUsers : IList<TUser>;
  end;

implementation

uses
  MetaADOConnectionUtils;

constructor TUserDBO.Create;
begin
  FDBSession := TDBConnection.GetInstance.CreateDBSessionManager;
end;

destructor TUserDBO.destroy;
begin
   FDBSession.Connection.Disconnect;
   FDBSession.Free;
   FDBSession := nil;
   inherited;
end;

function TUserDBO.GetAllUsers : IList<TUser>;
begin
   Result := FDBSession.FindAll<TUser>;
end;

and the call code on a form

var
  AllUsers : IList<TUser>;
  dao : TUserDBO;
  aUser : TUser;
begin
   dao := TUserDBO.Create;
   AllUsers := dao.GetAllUsers;
   Memo5.Lines.Clear;
      for aUser in AllUsers do
       Memo5.Lines.Add(aUser.LoginName)

The above codes raises above error but works all right if the Password Mapping which is a varbinary is commented out.

[Stefan Glienke]

Where does the exception come from? From Spring.TValueHelper.FromVariant? If so then the varbinary to string conversion needs to be added. I just don't know what the vartype of a varbinary field is from the top of my head.

[Stefan Glienke]

The ORM does not convert varbinary (which is internally treated as TArray<Byte>) to string by itself.

However using the TValueConverters you can register a converter that handles this. I recommend not using TArray<Byte> to string conversion but use a new string type for your password field to it only handles this explicit type.

type
  VarBinaryPasswordString = type string;

  [Table('TEST')]
  TTest = class
    [Column([cpPrimaryKey])]
    Id: Integer;
    [Column]
    Password: VarBinaryPasswordString;
  end;

Then define the converter and register it:

uses
  Spring.ValueConverters;

type
  TByteArrayToStringConverter = class(TValueConverter)
  protected
    function DoConvertTo(const value: TValue;
      const targetTypeInfo: PTypeInfo;
      const parameter: TValue): TValue; override;
  end;

function TByteArrayToStringConverter.DoConvertTo(const value: TValue;
  const targetTypeInfo: PTypeInfo; const parameter: TValue): TValue;
begin
  Result := TEncoding.Unicode.GetString(value.AsType<TArray<Byte>>); 
  // or another encoding depending on how the string is saved in the database
end;

  TValueConverterFactory.RegisterConverter(TypeInfo(TArray<Byte>),
    TypeInfo(VarBinaryPasswordString), TByteArrayToStringConverter);

You also need the other way around if you want to save the value. This is currently broken and I am fixing this.

[Stefan Glienke]

refactoring of TDBParam (fixed issue #119)

→ <<cset 19adefb97421>>