Question

I have two tables orders and orderdetails

table orders (PK = id, UNIQUE index on orderno)

|id|orderno|
| 1|1000   |
| 2|1001   |

table orderdetails (PK = id)

|id|orderid|item|qty|
| 1|      1|ABC |  3|
| 2|      1|XYZ |  4|

Now I want to query the data with:

SELECT o.orderno, od.item, od.qty
  FROM orders o

INNER JOIN orderdetails od ON o.orderno = od.order

which returns:

|orderno|item|qty|
|1000   |ABC |  3|
|1000   |XYZ |  4|

However If I use the following code to load the result into a DataTable it fails:

var connectionString = "Server=localhost;Database=orders;Uid=root;";
var commandText = "SELECT o.orderno, od.item, od.qty" + Environment.NewLine +
                  "FROM orders o" + Environment.NewLine +
                  "INNER JOIN orderdetails od ON o.orderno = od.order";

var reader = MySqlHelper.ExecuteReader(connectionString, commandText);
var table = new DataTable("OrdersQuery");
table.Fill(reader); // throws ConstraintException

The problem is, that

table.Constraints[0]

is a UniqueConstraints on the orderno column. Propably because

reader.GetSchemaTable() 

has a IsUnique=true entry for orderno (which is true in the base table, but not true for the join query).

Even worse, that doesn’t help either:

table.BeginLoadData(); // msdn docs claim that this should disable constraints
table.Load(reader);
table.EndLoadData();

Any ideas how to fix this?

StackTrace:

System.Data.ConstraintException Was Unhandled.
  Message=Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.
  Source=System.Data
  StackTrace:
       bei System.Data.DataTable.EnableConstraints()
       bei System.Data.DataTable.set_EnforceConstraints(Boolean value)
       bei System.Data.DataTable.EndLoadData()
       bei System.Data.Common.DataAdapter.FillFromReader(DataSet dataset, DataTable datatable, String srcTable, DataReaderContainer dataReader, Int32 startRecord, Int32 maxRecords, DataColumn parentChapterColumn, Object parentChapterValue)
       bei System.Data.Common.DataAdapter.Fill(DataTable[] dataTables, IDataReader dataReader, Int32 startRecord, Int32 maxRecords)
       bei System.Data.Common.LoadAdapter.FillFromReader(DataTable[] dataTables, IDataReader dataReader, Int32 startRecord, Int32 maxRecords)
       bei System.Data.DataTable.Load(IDataReader reader, LoadOption loadOption, FillErrorEventHandler errorHandler)
       bei System.Data.DataTable.Load(IDataReader reader)

Answer 1

I just figured out, that

table.Fill(reader)

does not create the constraint, if I already added the columns.

So I fixed this by using a nice little extension method:

    public static void Load(this DataTable table, IDataReader reader, bool createColumns)
    {

        if (createColumns)
        {
            table.Columns.Clear();
            var schemaTable = reader.GetSchemaTable();
            foreach (DataRowView row in schemaTable.DefaultView)
            {
                var columnName = (string)row["ColumnName"];
                var type = (Type)row["DataType"];
                table.Columns.Add(columnName, type);
            }
        }

        table.Load(reader);
    }

Usage:

table.Fill(reader, true);