GridView CommandArgument Needs Field From Repeater

I have a repeater. This gets bound to a data set which contains (amongst others) a field called EntityType and a field called ID. An Entity Type might be "Person" and there might be several categories of "Person". The ID field identifies that category. So there might be a "Person" type 1, a "Person" type 2 and so on. The Entity Type and ID therefore make up a compound primary key.

Nested in each row of the repeater I have a grid view which get populated as a result of clicking an "expand" button in the repeater row. Each row in the GridView represents an "Entity". So "Peter" who's own ID might be, say 59 might be a "Person" of Type 1. So Peter really has a three column primary key consisting of the EntityType and ID field's from the repeater's dataset and the ID from gridview's dataset.

So in order to query the database and fully retrieve Peter, I need that whole primary key. I know how to get the ID from the GridView's dataset, I set Eval("ID") as the CommandArgument for the gridview's button and then consume it in the event. But I don't know how to get at the EntityType or the ID from the Repeater's dataset. How can I do that?

Here's my code at the moment:-

asp Code:
<asp:Repeater ID="ChildClassesRepeater" runat="server" OnItemCommand="ChildClassesRepeater_ItemCommand" Visible ="false" >       
<ItemTemplate> <tr> <td> <%# DataBinder.Eval(Container.DataItem, "EntityType")%> </td> <td> 
<%# DataBinder.Eval(Container.DataItem, "Class")%> </td> <td>

[Code] ....

I'm thinking the easiest thing to do is put the EntityType and parent ID on the Command Argument where it currently reads IDontKnowWhatGoesHere but I can't work out how to get at them.

Alternatively, is there a way I can get at them in the event handler? I think not but I'm not sure.

I could have the query for the grid view return the parent columns, that would allow me to eval them in the same way as I am the entity ID. That feels like I'm reading redundant data, though, as it's already there in the client - if I could just get at it.