使用Entity Framework 4更新实体

Question

Here is the code I'm using to create a new Student.

public class StudentRepository
{
    SchoolEntities db = new SchoolEntities();

    public IQueryable<Student> FindAllStudents()
    {
        return db.Students;
    }

    public Student FindStudent(int id)
    {
        return db.Students.SingleOrDefault(c => c.ID == id);
    }

    public void Add(Student Student)
    {
        db.AddToStudents(Student);
    }

    public void Save()
    {
        db.SaveChanges();
    }
}

Here's how I'm using it:

private void SaveInformation()
{
    Student student = new Student();
    Int64 gradeId = Convert.ToInt64(cmbGradeParalelo.SelectedValue);
    student.IDGrade = gradeId;
    student.RUDE = Convert.ToInt64(txtRude.Text);

    /*Parents information.*/
    student.FatherName = txtNombrePadre.Text;
    student.FatherProfession = txtProfesionPadre.Text;
    student.MobilePhoneFather = FormatPhoneNumber(txtCelularPadre.Text);
    student.PlaceofWorkFather = txtLugarDeTrabajoPadre.Text;

    student.MotherName = txtNombreMadre.Text;
    student.MotherProfession = txtProfesionMadre.Text;
    student.MobilePhoneMother = FormatPhoneNumber(txtCelularMadre.Text);
    student.PlaceofWorkMother = txtLugarDeTrabajoMadre.Text;

    /*Student information*/
    student.Name = txtNombre.Text;
    student.FatherLastName = txtApellidoPaterno.Text;
    student.MotherLasteName = txtApellidoMaterno.Text;
    student.DateOfBirth = dtpFechaNacimiento.Value.ToShortDateString();
    student.PlaceOfBirth = txtLugarNacimiento.Text;
    student.Sex = sexoMasculino.Checked ? sexoMasculino.Text : sexoFemenino.Text;
    student.Telephone = FormatPhoneNumber(txtTelefono.Text);
    student.MobilePhone = FormatPhoneNumber(txtCelular.Text);
    student.Address = txtDireccion.Text;
    student.Carnet = FormatPhoneNumber(txtCarnet.Text);
    student.Observations = txtObservaciones.Text;

    StudentRepository repo = new StudentRepository();
    repo.Add(student);
    repo.Save();
    MessageBox.Show("Se guardo el registro exitosamente.",
        "Exito!",
        MessageBoxButtons.OK,
        MessageBoxIcon.Information,
        MessageBoxDefaultButton.Button1);
    ClearForm();
}

The problem is when I load the information on a form, and hit the save again, a new Student is created. I'd like to modify the students information.

Any suggestions?

Answer 1

You're creating a new Student instance yourself whenever you click save. That's correct if you want to create a new Student record, but if you want to edit an existing record, you will have to modify a Student record that you've retrieved from the database instead.

A better OO approach would be to have the calling form pass the Student object to your student editor form. This way, the calling form would instantiate a new Student object itself if creating a new one, then if the DialogResult of the form is OK , add it to the context and save it. In the case of opening an existing record, the calling form would pass the existing Student record to the editor, then call Save() once it closes.

On an unrelated note, you really need to Dispose of the context once you're done with it.

Answer 2

The easiest solution would be to get the Student first, then make the changes.

var existingStudent = StudentRepository.FindStudent(1); // object now in EF graph
student.RUDE = Convert.ToInt64(txtRude.Text); // object now set to EntityState.Modified
// other fields
repo.Save(); // object saved to database

On a side note, don't do this:

db.AddToStudents(Student);

The recommended way in EF4 is:

db.Students.AddObject(Student);

By the looks of the code, i'm guessing this is either a WinForms or WPF app? Maybe your better of using a control with EntityDataSource , as opposed to the painstaking task of manually updating the model.