Can't get multi-mapping to work in Dapper

Question

Playing around with Dapper, I'm quite pleased with the results so far - intriguing!

But now, my next scenario would be to read data from two tables - a Student and an Address table.

Student table has a primary key of StudentID (INT IDENTITY) , Address has an AddressID (INT IDENTITY) . Student also has an FK called AddressID linking into the Address table.

My idea was to create two classes, one for each table, with the properties I'm interested in. Additionally, I put an PrimaryAddress property of type Address onto my Student class in C#.

I then tried to retrieve both student and address data in a single query - I mimick the sample that's given on the Github page :

var data = connection.Query<Post, User>(sql, (post, user) => { post.Owner = user; });
var post = data.First();

Here, a Post and a User are retrieved, and the owner of the post is set to the user - the type returned is a Post - correct?

So in my code, I define two parameters to the generic Query extension method - a Student as the first which should be returned, and an Address as the second, which will be stored onto the student instance:

var student = _conn.Query<Student, Address>
                  ("SELECT s.*, a.* FROM dbo.Student s 
                        INNER JOIN dbo.Address a ON s.AddressID = a.AddressID 
                        WHERE s.StudentenID = @Id", 
                    (stu, adr) => { stu.PrimaryAddress = adr; },  
                    new { Id = 4711 });

Trouble is - I get an error in Visual Studio:

Using the generic method 'Dapper.SqlMapper.Query(System.Data.IDbConnection, string, System.Func, dynamic, System.Data.IDbTransaction, bool, string, int?, System.Data.CommandType?)' requires 6 type arguments

I don't really understand why Dapper insists on using this overload with 6 type arguments...

Answer 1

That would be cause I changed APIs and forgot to update the documentation, I corrected the error.

Be sure to have a look at Tests.cs for a full up-to-date spec.

In particular, the old API used to take in an Action<T,U> to perform the mapping, the trouble was that it felt both arbitrary and inflexible. You could not fully control the return type. The new APIs take in a Func<T,U,V> . So you can control the type you get back from the mapper and it does not need to be a mapped type.

I just tied up some additional flexibility around multi mapping, this test should make it clear:

class Person
{
    public int PersonId { get; set; }
    public string Name { get; set; }
}

class Address
{
    public int AddressId { get; set; }
    public string Name { get; set; }
    public int PersonId { get; set; }
}

class Extra
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public void TestFlexibleMultiMapping()
{
    var sql = 
@"select 
1 as PersonId, 'bob' as Name, 
2 as AddressId, 'abc street' as Name, 1 as PersonId,
3 as Id, 'fred' as Name
";
    var personWithAddress = connection.Query<Person, Address, Extra, Tuple<Person, Address,Extra>>
        (sql, (p,a,e) => Tuple.Create(p, a, e), splitOn: "AddressId,Id").First();

    personWithAddress.Item1.PersonId.IsEqualTo(1);
    personWithAddress.Item1.Name.IsEqualTo("bob");
    personWithAddress.Item2.AddressId.IsEqualTo(2);
    personWithAddress.Item2.Name.IsEqualTo("abc street");
    personWithAddress.Item2.PersonId.IsEqualTo(1);
    personWithAddress.Item3.Id.IsEqualTo(3);
    personWithAddress.Item3.Name.IsEqualTo("fred");

}

Dapper pipes all the multi mapping APIs through a single method, so if something fails it will end up in the 6 param one. The other piece of the puzzle was that I did not allow for some super flexible splits, which I just added.

Note, the splitOn param will default to Id , meaning it will take a column called id or Id as the first object boundary. However if you need boundaries on multiple primary keys that have different names for say a "3 way" multi mapping, you can now pass in a comma separated list.

So if we were to fix the above, probably the following would work:

 var student = _conn.Query<Student,Address,Student>
              ("SELECT s.*, a.* FROM dbo.Student s 
                    INNER JOIN dbo.Address a ON s.AddressID = a.AddressID 
                    WHERE s.StudentenID = @Id", 
                (stu, adr) => { stu.PrimaryAddress = adr; return stu;},  
                new { Id = 4711 }, splitOn: "AddressID").FirstOrDefault();