Setting Columns to Read-Only in a Union Query: A Guide for .NET Developers

Understanding DataTable Columns as ReadOnly when Using a Union

The question posed by the original poster highlights an issue with setting columns to readonly in a DataTable that is created through a union query. The resulting SQL does indeed indicate that the columns are being marked as readonly, but the actual behavior of the DataTable itself may differ from this representation.

Background and Context

In order to fully understand the problem at hand, it’s necessary to delve into some of the underlying concepts and processes involved in creating a DataTable. A DataTable is a powerful tool in .NET for handling tabular data. The data can be read and written through various methods such as adding rows, inserting new columns, and performing updates.

When creating a DataTable, developers often use SQL queries to fetch the data. These SQL queries are then used by the DataTable to populate its contents. In many cases, these SQL queries contain joins, unions, or other complex operations that can make it difficult for the DataTable to determine whether certain columns should be readonly.

Technical Terms and Processes

Union: A union operation combines two or more SQL queries into a single query. This is exactly what’s happening in this case.
Datatable : A powerful object in .NET that allows you to manipulate and manage tabular data.
SqlGenerator: An internal class in .NET that generates SQL queries based on the provided parameters.

The Problem with UNION

The problem arises because the SQL query being used by the DataTable is a union operation. When this happens, the DataTable tries to determine whether certain columns are readonly or not. However, due to the complexity of the SQL query, it’s difficult for the DataTable to accurately assess which columns should be readonly.

The answer provided highlights that one way to solve this issue is to set all columns to writable and provide an update query manually to the builder/adapter. Alternatively, if possible, using the dataset designer and creating a new dataset that is designed and strongly typed to represent what you need can also help avoid this problem.

Step-by-Step Solution

One approach to solving this problem is to create two separate DataTables for each part of the union query, rather than trying to use a single union operation. This way, you can ensure that each DataTable has an updatable dataset with the correct columns.

Here’s an example of how you could do it:

// Define a function that creates a DataTable from an SQL query
public DataTable GetDataTable(string commandText)
{
    var da = new DataTable();
    // ...
    return da;
}

// Create two separate DataTables for each part of the union query
var internalPositions = GetDataTable("SELECT * FROM [PositionReconciliation.InternalPosition]");
var externalPositions = GetDataTable("SELECT * FROM [PositionReconciliation.ExternalPosition]");

// Perform a UNION operation on these two DataTables
var result = internalPositions.Copy();
foreach (DataRow row in externalPositions.Rows)
{
    var newRow = internalPositions.NewRow();
    // Map columns from externalPositions to newRow
    // ...
    result.Rows.Add(newRow);
}

// Return the resulting DataTable
return result;
}

Code Example

public IgnoredPositionSet GetAllForReconciliation(int aReconciliationId)
{
    SqlGenerator internalIgnoredPositionSqlGenerator = base.GetCoreSqlGenerator()
        .Select.Add(@"
          IP.SecurityId,
          IP.SecurityName,
          IP.Quantity,
          NULL AS AccountTypeName")
        .Join.Add($@"
          LEFT JOIN 
            (
              SELECT
                ReconciliationId,
                MatchId,
                SecurityId,
                SecurityName,
                SUM(QuantityTradeDate) AS Quantity
              FROM [PositionReconciliation.InternalPosition]
              GROUP BY
                ReconciliationId,
                MatchId,
                SecurityId,
                SecurityName
            ) IP
          ON IP.ReconciliationId = {ServiceTableAlias}.ReconciliationId
          AND IP.MatchId         = {ServiceTableAlias}.MatchId")
        .Where.Add("IsInternalPosition = 1");

    SqlGenerator externalIgnoredPositionSqlGenerator = GetCoreSqlGenerator()
        .Select.Add(@"
          NULL AS SecurityId,
          EP.SecurityName,
          EP.Quantity,
          AccountTypeName")
        .Join.Add($@"
          LEFT JOIN 
            (
              SELECT
                ReconciliationId,
                MatchId,
                SecurityName,
                SUM(QuantityTradeDate) AS Quantity
              FROM [PositionReconciliation.ExternalPosition]
              GROUP BY
                ReconciliationId,
                MatchId,
                SecurityName
            ) EP
          ON EP.ReconciliationId = {ServiceTableAlias}.ReconciliationId
          AND EP.MatchId         = {ServiceTableAlias}.MatchId")
        .Join.Add($@"
          LEFT JOIN
            (
              SELECT
                EP.ReconciliationId,
                EP.MatchId,
                A.AccountTypeId,
                AT.Name AS AccountTypeName
              FROM [PositionReconciliation.ExternalPosition] EP
              LEFT JOIN [Core.CustodianFund.AccountMap]      A  ON A.Id  = EP.CustodianAccountId
              LEFT JOIN [Reconciliation.Cash.AccountType]    AT ON AT.Id = A.AccountTypeId
              GROUP BY ReconciliationId, MatchId, AccountTypeId, AT.Name
            ) EPAT
          ON  EPAT.ReconciliationId = {ServiceTableAlias}.ReconciliationId
          AND EPAT.MatchId          = {ServiceTableAlias}.MatchId")
        .Where.Add("IsInternalPosition = 0");

    var internalPositions = GetEntitySet(internalIgnoredPositionSqlGenerator); // ReadOnly props are false
    var externalPositions = GetEntitySet(externalIgnoredPositionSqlGenerator); // ReadOnly props are false

    return GetEntitySet($"SELECT * FROM ({internalIgnoredPositionSqlGenerator} UNION ALL {externalIgnoredPositionSqlGenerator}) {ServiceTableAlias} WHERE {nameof(IgnoredPosition.ReconciliationId)} = {aReconciliationId}");
}

Conclusion

Setting columns to readonly in a DataTable that is created through a union query can be tricky. In most cases, the best approach is to create two separate DataTables for each part of the union query and then perform a UNION operation on these two DataTables. This way, you can ensure that each DataTable has an updatable dataset with the correct columns.

Remember, using strongly typed data containers and avoiding complex SQL operations can help prevent this problem altogether.

Last modified on 2023-07-11