ADO.NET: Adding DataRelation to a DataSet; which is parent and which is child?

ID   ParentID   Text
===  =========  =============
1    (null)     Product
2    (null)     Applications
3    1          Background
4    1          Details
5    2          Mobile

public DataRelation(
    string relationName,
    DataColumn parentColumn,
    DataColumn childColumn,
    bool createConstraints
)

DataRelation relation = newDataRelation("ParentChild",
    ds.Tables[0].Columns["ID"], //parentColumn
    ds.Tables[0].Columns["ParentID"] //childColumn,
    true //createConstraints
);

This constraint cannot be enabled
as not all values have corresponding parent values.

DataRelation relation = newDataRelation("ParentChild",
    ds.Tables[0].Columns["ParentID"], //parentColumn
    ds.Tables[0].Columns["ID"], //childColumn
    true //createConstraints
);

relation.Nested = true;
ds.Relations.Add(relation);