ASP.NET dataset add relation null reference exception - asp.net

Question

I'm using this article as a guide to doing a nested data bind of my two tables 'Comment' 'CommentOtherAuthor': http://support.microsoft.com/kb/306154. There may be many authors to one comment. The code I have is here:
.ASPX:
<asp:Repeater ID="rptComments" runat="server">
<ItemTemplate>
<div class="comment-data">
<h3 class="item">Submitted to <%# GetPageDetails(Eval("nodeid")) %> article on <%# Eval("created") %></strong></h3>
<p class="item"><strong>Name</strong> <%# Eval("firstname") %> <%# Eval("surname") %></p>
<p class="item"><strong>Occupation</strong> <%# Eval("occupation") %></p>
<p class="item"><strong>Affiliation</strong> <%# Eval("affiliation") %></p>
<p class="item"><strong>Email</strong> <a href='mailto:<%# Eval("email") %>'><%# Eval("email") %></a> <em>Publish email: <%# Eval("publishemail") %></em></p>
<p class="item"><strong>Competing interests?</strong> <%# Eval("competingintereststext") %> </p>
<p class="item"><strong>eLetter title</strong> <%# Eval("title") %></p>
<p><%# Eval("comment").ToString().Replace("\n", "<br/>")%></p>
<div class="additional-authors">
<h3>Additional authors</h3>
<asp:Repeater id="rptAdditionalAuthors" runat="server" datasource='<%# ((DataRowView)Container.DataItem).Row.GetChildRows("myrelation") %>' >
<ItemTemplate>
<%# DataBinder.Eval(Container.DataItem, "[\"firstname\"]")%><br>
</ItemTemplate>
</asp:Repeater>
</div>
</div>
</ItemTemplate>
</asp:Repeater>
Codebehind:
private void BindData()
{
SqlConnection cnn = new SqlConnection(GlobalSettings.DbDSN);
SqlDataAdapter cmd1 = new SqlDataAdapter(string.Format("select * from Comment {0} order by created desc", Filter), cnn);
//Create and fill the DataSet.
DataSet ds = new DataSet();
cmd1.Fill(ds, "comments");
//Create a second DataAdapter for the additional authors table.
SqlDataAdapter cmd2 = new SqlDataAdapter("select * from CommentOtherAuthor", cnn);
cmd2.Fill(ds, "additionalAuthors");
//Create the relation between the comments and additional authors tables.
ds.Relations.Add(
"myrelation",
ds.Tables["Comment"].Columns["id"],
ds.Tables["CommentOtherAuthor"].Columns["commentid"]
);
//Bind the Authors table to the parent Repeater control, and call DataBind.
rptComments.DataSource = ds.Tables["additionalAuthors"];
rptComments.DataBind();
}
However, when running this it throws a System.NullReferenceException on the line ds.Relations.Add(
I'm really not sure where to start in fixing this as I'm way out of my depth here.
Can anyone advise how to get this working?
Thanks.

Answer 1

Got it! There was no actual foreign key constraint joining the tables. With that in place this now works.