primary keys

When i use the code below to add a datarelation to 2 tables named 'Category' and 'Product' and then try to find a row within the tables using the find method shown below I get the error 'Table doesnt have a Primary Key'. DataRelation catAndProdDataRelation = new DataRelation("CategoryAndProductDataRelation", ds.Tables["Category"].Columns[0] , ds.Tables["Product"].Columns[3], true); ds.Relations.Add(catAndProdDataRelation); DataRow cdr = ds.Tables["Category"].Rows.Find(id); I have also tried adding the primary keys explicitly with ds.Tables["Category"].Constraints.Add("PK_ColumnID", ds.Tables["Category"].Columns[0], true); ds.Tables["Product"].Constraints.Add("PK_ProductID", ds.Tables["Product"].Columns[0], true); I get the error constraint matches constraint 1 already in collection I thought the datarelation created the primary keys and the constraint matches error seems to indicate that. Any help much appreciated