Unable to cast object of type 'System.Data.Objects.ObjectQuery`1 ..... to type 'DatabaseModel1......'. ASP

Question

Hi im trying to assign a default group to any user that registers with my site. Since the admin Id of that group is the administrator, I query my database for a group with a admin id of my administrator and then assign that group to the user. However the following error is being thrown "Unable to cast object of type 'System.Data.Objects.ObjectQuery`1[DatabaseModel1.Group]' to type 'DatabaseModel1.Group'. " Heres the code

        Dim defaultGroup = (From group As Group In context.Groups
                     Where group.AdminID = ((From users As User0 In context.User0
                                            Where users.Name Like "Administrator"
                                            Select users.UserID).First)
                       Select group)

        currentUser.Groups.Add(defaultGroup)//the error is being thrown here

any help would be appreciated Thanks

Jon Skeet · Accepted Answer · 2012-03-04 16:39:06Z

5

It sounds like you've got a query which can return multiple results, but you're trying to assign it to a single-value variable.

You probably just need to use something like:

currentUser.Groups.Add(defaultGroup.First)

The probable options are:

First() - allows multiple results and returns the first; will throw an exception if there are none
FirstOrDefault() - allows multiple results and returns the first; will return the default value for the element type (e.g. null) if there are no results
Last() - allows multiple results and returns the last; will throw an exception if there are none
LastOrDefault() - allows multiple results and returns the last; will return the default value for the element type (e.g. null) if there are no results
Single() - expects exactly one result; if there are no results or mulitple results, an exception is thrown

Now that you've shown the query, it sounds like you should probably use a join rather than a nested query. I'm not hot on the VB query syntax, but in C# you might want:

var defaultGroupQuery = 
        from group in context.Groups
        join user in context.User0 on group.AdminID equals user.UserID
        where user.Name == "Administrator"
        select group;

answered Mar 4, 2012 at 16:39

Jon Skeet

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1 Comment

Simon Over a year ago

Damn... can't believe I fell for this for a second time. Thanks Jon!

Matija Grcic · Accepted Answer · 2012-03-04 17:07:32Z

    List<int> adminIDs = context.User0.Where(u=> u.Name.Contains("Administrator"))
                               .Select(u=>u.UserId);

    //you can use `FirstOrDefault()` above and it will return only int not `List<int>`
    // but i'm showing you how to get all IDs then you can get the first one

    int adminId = adminIDs.FirstOrDefault();

    //and then get only one Group using this adminID
    var defaultGroup = context.Groups.FirstOrDefault(g=>g.AdminID == adminId);
    if(defaultGroup != null)
    {
      currentUser.Groups.Add(defaultGroup);
    }

    //you can use the `List<int> adminIDs` if you need to get all admin groups

var groups = context.Groups.Where(gg => adminIDs.Contains(gg.AdminID));

Hope it helps.

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