Saving nested objects with linq to entities

Question

I have a UI that presents a number of checkboxes to the user, and for each one that is checked, I need to create an entry in a mapping table.

LookupTable <- MapTable -> DataTable

I have a custom binder for DataTable, but can't figure out how to get it to create the MapTable objects.

Is this possible? Using asp.net MVC 1.0 and LINQ to Entities.

Answer 1

You need to take care of two steps. (1) Add the newly selected values and (2) remove the unselected values. You'll need a method like this in your LookupTable class.

public void SynchronizeDataTables(IEnumerable<DataTable> dataTables)
{
    // get the current data tables. call ToList() to force and enumeration.
    // without the ToList(), you'll get a "Sequence Changed during Enumeration"
    // error
    var currentDataTables = MapTable.Select(m => m.DataTable).ToList();

    // if the table is selected, but not in the data store add it.
    foreach (var dataTable in dataTables)
    {
        if (!currentDataTables.Contains(dataTable))
        {
            MapTables.Add(new MapTable { DataTable = dataTable });
        }
    }

    // if the table is in the data store, but not selected, then remove it.
    foreach (var dataTable in currentDataTables)
    {
        if (!dataTables.Contains(dataTable))
        {
            MapTables.Remove(dataTable);
        }
    }
}

Edit: When I did this, I was using LINQ-to-SQL, and I cycled through just the selected ID's, instead of the entire object. This is more complicated because LINQ-to-Entities creates slightly different objects than LINQ-to-SQL, because it doesn't expose the FK identity. Slight modification follows:

public void SynchronizeDataTables(IEnumerable<int> dataTableIds)
{
    // get the current data tables. call ToList() to force and enumeration.
    // without the ToList(), you'll get a "Sequence Changed during Enumeration"
    // error
    var currentDataTableIds = MapTable.Select(m => m.DataTable.Id).ToList();

    // if the table is selected, but not in the data store add it.
    foreach (var dataTableId in dataTableIds)
    {
        if (!currentDataTableIds.Contains(dataTableId))
        {
            var dataTable = ???; // some method to fetch data table with ID = dataTableId
            MapTables.Add(new MapTable { DataTable = dataTable });
        }
    }

    // if the table is in the data store, but not selected, then remove it.
    foreach (var dataTable in currentDataTableIds )
    {
        if (!dataTableIds.Contains(dataTableId ))
        {
            var dataTable = ???; // some method to fetch data table with ID = dataTableId
            MapTables.Remove(dataTable);
        }
    }
}