updating category id for the product instead of creating new id

i have two tables

product

                product_id
                product_Name
    开发者_开发技巧            product_Price
                product_Description
                product_image
                category_id

another table

                 category
                 category_id
                 category_name
                 category_description

i have a form with three text boxes(say tbProductPrice,tbProductName,tbProductdescription) one combo box(cbcategorytypes) two buttons one edit and another one is save button.. i am trying update the product table along with category_id

when i click the edit button the category names are loaded in the combobox

when we click on the save button what ever values in text boxes will be updated in product table along with category for that i have done the below code...

              using (var vareditcontext = new abcEntities())
            {
                pictureBox1.Enabled = true;
                pictureBox1.Visible = true;
                Image image = pictureBox1.Image;
                byte[] bit = null;

                bit = imageToByteArray(image);
                product1 pd = vareditcontext.product1.Where(p => p.product_Id == productid
                                     && p.category_Id == productcategoryid).First();


                string category = cbcategorytypes.Text;

                var c = new category { category_Name = category }; //problem at this line 

                pd.category = c;
                pd.product_Name = tbProductName.Text;
                decimal price = Convert.ToDecimal(tbProductPrice.Text);
                pd.product_Price = price;
                pd.product_Description = tbProductdescription.Text;
                pd.product_Image = bit;
                vareditcontext.SaveChanges();                 
                this.Close();
            }

when i click the save button i got an exception like this ..

Argument out of range exception ..

I am getting this error because when i edit and trying to save the product details along with category name the new category name will be stored in database.....instead of updating the present one ...

how i can rectify this problem .. i mean not storing the new item i want to set the already exsiting category to the product...

is it possible with linq ....

would any one help on this..

many thanks....

I assume that category_id is the Key property for your category entity. If you only load the category_name into the combobox you actually need to load the Category from the database because EF must know the key value when you assign the category to the product:

product1 pd = vareditcontext.product1
    .Where(p => p.product_Id == productid
        && p.category_Id == productcategoryid)
    .First();

category c = vareditcontext.categories
    .Where(cat => cat.category_name == cbcategorytypes.Text)
    .First(); // let's hope the name is unique to pick not the wrong one

pd.category = c;
// ...
vareditcontext.SaveChanges();

You could also only load the category_id and then leverage Dennis' approach to create a stub category and attach it to the context:

product1 pd = vareditcontext.product1
    .Where(p => p.product_Id == productid
        && p.category_Id == productcategoryid)
    .First();

int categoryid = vareditcontext.categories
    .Where(cat => cat.category_name == cbcategorytypes.Text)
    .Select(cat => cat.category_id)
    .First();

var c = new category { category_id = categoryid };
    // stub entity must have at least the key property set
vareditcontext.categories.Attach(c);

pd.category = c;
// ...
vareditcontext.SaveChanges();

If you would expose the category_id column in the product table as a foreign key property into your product model class you could just set the category_id:

product1 pd = vareditcontext.product1
    .Where(p => p.product_Id == productid
        && p.category_Id == productcategoryid)
    .First();

int categoryid = vareditcontext.categories
    .Where(cat => cat.category_name == cbcategorytypes.Text)
    .Select(cat => cat.category_id)
    .First();

pd.category_id = categoryid;
// ...
vareditcontext.SaveChanges();

You need to either load the category from the context or attach the one you create on the fly to the context. Otherwise EF assumes you want to create and store a new one.

...
var c = new category { category_Name = category };
vareditcontext.Attach(c);
pd.category = c;
...

At MSDN, you can read more about Attaching and Detaching Objects.