Question

var sql = @"SELECT
    a.id AS `Id`, 
    a.thing AS `Name`, 
    b.id AS `CategoryId`,
    b.something AS `CategoryName`  
FROM ..";

var products = connection.Query<Product, Category, Product>(sql,
    (product, category) =>
    {
        product.Category = category;
        return product;
    }, 
    splitOn: "CategoryId");

foreach(var p in products)
{
    System.Diagnostics.Debug.WriteLine("{0} (#{1}) in {2} (#{3})", p.Name, p.Id, p.Category.Name, p.Category.Id);
}

Results in:

'First (#1) in  (#0)'
'Second (#2) in  (#0)'

CategoryId and CategoryName has values since the following

var products = connection.Query(sql).Select<dynamic, Product>(x => new Product
{
    Id = x.Id,
    Name = x.Name,
    Category = new Category { Id = x.CategoryId, Name = x.CategoryName }
});

Results in:

'First (#1) in My Category (#10)'
'Second (#2) in My Category (#10)'

I’m connecting to a MySQL database if that has anything to do with it.

Answer 1

The simplest way is to call them all Id (case-insensitive, so just a.id and b.id are fine); then you can use:

    public void TestMultiMapWithSplit()
    {
        var sql = @"select 1 as Id, 'abc' as Name, 2 as Id, 'def' as Name";
        var product = connection.Query<Product, Category, Product>(sql,
           (prod, cat) =>
           {
               prod.Category = cat;
               return prod;
           }).First();
        // assertions
        product.Id.IsEqualTo(1);
        product.Name.IsEqualTo("abc");
        product.Category.Id.IsEqualTo(2);
        product.Category.Name.IsEqualTo("def");
    }

If you can’t do that, there is an optional splitOn (string) parameter that takes a comma-separated list of columns to treat at splits.