Setting a Lookupfield for current Item in List

Question

my use-case:

I'm trying to implement an event-management system to sharepoint using visual web part(s).

I have two custom lists, one is called "events". Let's say it has a column "Title" which shows the Event Details in a Displayform (the out-of-the-box title field it is, actually) and some other non-relevant details in other columns.

The second List is called "Attendees". There are two columns, first: "Attendee" (user field getting the loginname of the current user). Second: "Event" (at the moment: String of the Event title).

At my visual webpart (added via &ToolPaneView=2 above display form of the "Events"-List) I show buttons ("register" and "unregister") depending on the following code:

...
using System.Linq;
...
//Snippet of prerequisites for better understanding
SPWeb web = SPContext.Current.Web;
SPUser currentUser = web.CurrentUser;
SPListItem currentEventItem = (SPListItem)SPContext.Current.Item;
string userString = currentUser.ID.ToString() + ";#" + currentUser.LoginName.ToString();
...
//called in page_load
 private void GetButtonView()
    {
        try
        {
            bool foundUsers = (from SPListItem item in tn.Items
                               where item["Attendee"].ToString() == userString
                               && item["Event"].ToString() == currentEventItem.Title
                               select item).Count() > 0;
            if (foundUsers)
            {
                ...
                ButtonRegister.Visible = false;
            }
            else
            {
                ...
                ButtonUnregister.Visible = false;
            }
        }
        catch (Exception ex)
        {
            ...
        }
    }

Problem at this use-case: If the event title ("Title") is changed later on in the events-list, I don't get a correct mapping attendee => event because at the moment I check only for matching of title-strings.

Preferred Solution: Make the "Event" Column at the Atendees-List a lookup column which gets the actual Event (ID and the title, I think) and shows the title in the "Attendees" List "Event"-column, so if the title is changed later on in the "Events"-List, the attendees-list will automatically update the referring entries in its "Event"-Column and I can show the "Register"/"Unregister"-Button (apart from having a correct mapping, which is necessary for sure ;) ).

It would be great if someone could give me a solution/hint how to do this programmatically for I don't get it to work.

Best regards, Dominik

p.s. I am german so I hope you understand my question. Feel free to ask for more details! I'll try my best to explain it in proper english.

edit 1: Here's my so-far addAttendee-method, too:

private void AddAttendee() {
        try
        {
            //Using this because read-permissions are used for attendees
            SPSecurity.RunWithElevatedPrivileges(delegate()
            {
                using (SPSite ElevatedSite = new SPSite(site.ID))
                {
                    using (SPWeb ElevatedWeb = ElevatedSite.OpenWeb(web.ID))
                    {
                        SPList attendeeList= ElevatedWeb.Lists["Attendees"];
                        SPListItemCollection listItems = attendeeList.Items;
                        SPListItem item = listItems.Add();
                        item["Name"] = currentUser.ID.ToString() + ";#" + 
                        currentUser.LoginName.ToString() + ";#";
                        item["Event"] = currentEventItem["Title"];
                        ElevatedWeb.AllowUnsafeUpdates = true;
                        item.Update();
                        ElevatedWeb.AllowUnsafeUpdates = false;
                        ...
                    }
                }
            });
        }
        catch (Exception ex) 
        {
            ...
        }
    }

Answer 1

As far as I know, changing field type from text to lookup is not supported in Sharepoint.

You should create new lookup field in the Attendees list and point it to Events list, either from Sharepoint list settings, or from code.

Basically, lookup field has the same structure as user field, which you populated in the final bit of your code - ID#;Title. Also, lookup field value is represented by SPFieldLookupValue class, so it can be retrieved like that:

return new SPFieldLookupValue((string)item["EventLookup"]);

You can set lookup field value by assigning new SPFieldLookupValue object to SPListItem property:

item["EventLookup"] = new SPFieldLookupValue(1, "Great Event");

where 1 - Event list item Id and "Great Event" - Event list Item title.

If I also may add some remarks to your code examples. As far as I can see, you are searching attendees for the event by iterating through SPList.Items object and comparing a string value of "Attendee" field with custom string value.

Such a search can return wrong and slow results, because usually user lookup field contains user display name, not a Login Name. I'd recommend using CAML query instead:

SPQuery qry = new SPQuery();
qry.Query =
"  <Where>"
+"    <And>"
+"      <Eq>"
+"          <FieldRef Name='Event' />"
+"          <Value Type='Text'>"+ currentEventItem.Title +"</Value>"
+"      </Eq>"
+"      <Eq>"
+"          <FieldRef Name='Attendee' LookupId='TRUE' />"
+"          <Value Type='Integer'>"+ currentUser.ID.ToString() +"</Value>"
+"      </Eq>"
+"    </And>"
+"  </Where>";
SPListItemCollection listItems = spList.GetItems(qry);
bool foundUsers = listItems.Count > 0;

The query above searches by Event field (type "text" as in your example) and by Attendee user Id without user login Name. Also it retrieves only items, that fall whithin the query where clause.