.NET Core web API + Dapper – MultiMapping

Shaping recordsets with SplitOn

Sometimes there is no easy way to build a “list of lists” using the contents of multiple database tables. I have run into this problem a few times especially when building webby services which query the database of a “legacy” (aka. production) system.

Retrieving a list of StockGroups and their StockItems from the World Wide Importers database was one of the better “real world” examples I could come up with.

SQL Server Management Studio Diagram showing relationships of tables

There is a fair bit of duplication (StockGroupID, StockGroupName) in the results set

SQL Server Management Studio StockItems-StockItemStockGroups-StockGroups query and results

There were 442 rows in the results set and 227 StockItems in the database so I ordered the query results by StockItemID and confirmed that there were many StockItems in several StockGroups.

public class StockItemListDtoV1
	public int Id { get; set; }

	public string Name { get; set; }

	public decimal RecommendedRetailPrice { get; set; }

	public decimal TaxRate { get; set; }

public class StockGroupStockItemsListDto
		StockItems = new List<StockItemListDto>();

	public int StockGroupID { get; set; }

	public string StockGroupName { get; set; }

	public List<StockItemListDto> StockItems { get; set; }

My initial version uses a Generic List for a StockGroup’s StockItems which is most probably not a good idea.

public class InvoiceQuerySplitOnController : ControllerBase
	private readonly string connectionString;
	private readonly ILogger<InvoiceQuerySplitOnController> logger;

	public InvoiceQuerySplitOnController(IConfiguration configuration, ILogger<InvoiceQuerySplitOnController> logger)
		this.connectionString = configuration.GetConnectionString("WorldWideImportersDatabase");

		this.logger = logger;

	public async Task<ActionResult<IAsyncEnumerable<StockGroupStockItemsListDto>>> Get()
		IEnumerable<StockGroupStockItemsListDto> response = null;

			using (SqlConnection db = new SqlConnection(this.connectionString))
				var stockGroups = await db.QueryAsync<StockGroupStockItemsListDto, StockItemListDto, StockGroupStockItemsListDto>(
					sql: @"SELECT [StockGroups].[StockGroupID] as 'StockGroupID'" +
								",[StockGroups].[StockGroupName]" +
								",[StockItems].StockItemID as 'ID'" +
								",[StockItems].StockItemName as 'Name'" +
								",[StockItems].TaxRate" +
								",[StockItems].RecommendedRetailPrice " +
							"FROM [Warehouse].[StockGroups] " +
							"INNER JOIN[Warehouse].[StockItemStockGroups] ON ([StockGroups].[StockGroupID] = [StockItemStockGroups].[StockGroupID])" +
							"INNER JOIN[Warehouse].[StockItems] ON ([Warehouse].[StockItemStockGroups].[StockItemID] = [StockItems].[StockItemID])",
					(stockGroup, stockItem) =>
						// Not certain I think using a List<> here is a good idea...
						return stockGroup;
				splitOn: "ID",
				commandType: CommandType.Text);

			response = stockGroups.GroupBy(p => p.StockGroupID).Select(g =>
				var groupedStockGroup = g.First();
				groupedStockGroup.StockItems = g.Select(p => p.StockItems.Single()).ToList();
				return groupedStockGroup;
	catch (SqlException ex)
		logger.LogError(ex, "Retrieving S, Invoice Lines or Stock Item Transactions");

		return this.StatusCode(StatusCodes.Status500InternalServerError);

	return this.Ok(response);

The MultiMapper syntax always trips me up and it usually takes a couple of attempts to get it to work.

List of StockGroups with StockItems

I have extended my DapperTransient module adding WithRetry versions of the 14 MultiMapper methods.

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