MultiMap does not return null object in left-outer-join if splitting on a surrogate key that is not a property on the target object.

This issue is really for left-outer-joins where we would expect a null object if there is no match. Unfortunately, the object is not coming back null even though the 'spliton' column is null.

Class structure
public class Post[PostID,Name,CategoryID?] //category is optional (left-outer joined)
public class Category[CategoryID,Name]

Query using aliases to split on 'id'

SELECT p.*, c.CategoryID AS id, //surrogate key helps mark the split...will be null if no matching category c.* FROM Posts p LEFT OUTER JOIN Categories c ON c.CategoryID = p.CategoryID

If there is no category, the expected behavior is that because the 'id' column is null the entire object would be null. However, because 'id' does not exist as a property on the Category object we get an instantiated object.

Note that we're splitting this way because p.* already has CategoryID so we can't explicitly split on that.

I've gone through the source code for a bit but it's a little beyond me (for now at least). I'll keep trying to pinpoint why that is happening and get a test in there to prove it.

Start at Dapper's MultiMap handling for the splitOn alias in the shown left-outer-join query, and reproduce the case where the aliased id is null but is not a property on Category. Trace how the split column determines whether the target object is created. Done means an unmatched Category maps to null, with a regression test covering this query shape.