DropDownListFor Composite Key with Value Column - asp.net-mvc-4

Question

I'm building a Roles/Permissions component for my application. My Role view model looks like this:
public class RoleView
{
public int Id { get; set; }
public ApplicationWithPermissionsView Application { get; set; }
public List<RolePermissionView> Permissions { get; set; }
public IEnumerable<KeyValuePair<string, int>> PermissionValues
{
get
{
return EnumHelper.ToList<PermissionValue>();
}
}
[Required]
public string Name { get; set; }
}
The RolePermission view model looks like this:
public class RolePermissionView
{
public int PermissionId { get; set; }
public int RoleId { get; set; }
public PermissionValue Value { get; set; }
}
PermissionValue is an enum that has members NotSet, Allow, Deny, and stores an int value in the database.
If I go to a role edit page, I display a list of permission categories, which each have a list of permissions, with the respective values (RolePermission) for each permission for that role. As you can see, RolePermissionView has composite keys (PermissionId, RoleId).
How can I build a drop down list, that shows a list of possible permission values and selects the correct permission value for that particular permission?
The following generates the drop down lists, but doesn't select any values. Also, doesn't give each dropdown list a unique name or ID, so binding definitely won't work. I assume if I later move this into an Editor Template they will have uniques.
#foreach (var permission in category.Permissions)
{
<tr>
<td>#permission.Description</td>
<td>#Html.DropDownListFor(m => m.Permissions.Find(p => p.PermissionId == permission.Id).PermissionValue, new SelectList(Model.PermissionValues, "Value", "Key"))</td>
</tr>
}

Answer 1

The trouble here is mostly syntactic. Bear in mind that for things like DropDownListFor and the other *For-style helpers, you're passing an expression, not a literal instance of something. The helper then attempts to parse that expression to create a name attribute value on the input that will (hopefully) bind when the data is posted back. Obviously, then, doing something like Find is not going to work, because there's no way to translate that so that the same thing happens on the other end after posting. Instead, you should do something like:
#for (var i = 0; i < category.Permissions.Count(); i++)
{
<tr>
<td>#category.Permissions[i].Description</td>
<td>#Html.DropDownListFor(m => category.Permissions[i].PermissionValue, Model.PermissionValues)</td>
</tr>
}
Now, the expression is concrete enough to bind to.
Also, Change your PermissionValues property to be IEnumerable<SelectListItem> instead of using KeyValuePair. Then, there's no need to manually create a SelectList, which will save you a lot of potential headaches. To construct the Value of the SelectListItem, I would simply append the two composite ids as strings (since the value must be a string anyways). Join them by some delimiter like a comma. Then, on post, you can get the posted value and split it by the same delimiter to get your two ids back. Obviously, you'll also then need to convert them both back to ints. You can only post one value back from a select, so this is really your only option when it comes to composite keys.