要在对象，精简程序中列出的多重映射字符串

Question

I'm all new to Dapper and I'm trying to get my head around how it actually maps things. I have the following database structure:

calendar
| Id | Name |

meeting_room
| Id | Calendar_id | Capacity |

meeting_room_purpose
| id | Name | Description |

meeting_room_meeting_room_purpose
| id | room_id | purpose_id |

The 'calendar' table is much like an account. It can have multiple meeting rooms associated with it. A meeting room can have multiple purposes, which is the predicament of my question. I need to have an object like this:

class MeetingRoom {
     public string Name { get; set; }
     public int Capacity { get; set; }
     public List<string> Purpose { get; set; }
}

So from calendar, I get the name, meeting_room the capacity and the list of purposes from all tuples with a purpose for the given room. My query is like this:

SELECT  
    calendar.id as CalendarId,
    calendar.name as name,
    meeting_room.capacity as capacity,
    purpose
FROM calendar
LEFT JOIN meeting_room ON meeting_room.calendar_id=calendar.id
INNER JOIN 
    (SELECT 
         meeting_room_purpose.name as purpose
     FROM
         meeting_room_purpose
     INNER JOIN meeting_room_meeting_room_purpose mrmrp ON
                meeting_room_purpose.id=mrmrp.purpose_id 
                AND mrmrp.room_id=@Id)
         a
     WHERE meeting_room.id=@Id    

The output of this query is a set of tuples, which differ only in purpose, eg

| CalendarId |      name      | capacity |        purpose        |
      53         Charmander         6         Internal Meetings
      53         Charmander         6         Marketing Meetings 
      53         Charmander         6         Sales Meetings   

So, the meeting room "Charmander", has a capacity of 6 people, and can be used for 3 purposes. It should map to the object as (had it had a simple constructor):

new MeetingRoom("Charmander", 6, new List<string>(){"Internal Meetings", "Marketing Meetings", "Sales Meetings"); 

My dapper query is as:

_connection.Query<MeetingRoom, string, MeetingRoom>(sql, 
    (room, purpose) => { 
        room.Purpose = purpose; return room; 
    }, 
    new { Id = query.Id }, splitOn: "id, purpose").SingleOrDefault();

If I remove splitOn i get an exception, and if I keep it I just get an empty (null) object. I'm sure it's not too hard, I just can't seem to get my "eurika" moment.

Best Regards

Answer 1

Dapper's multimapping feature isn't really designed for this One to Many style mapping. This SO Answer shows a way to load the rooms and purposes separately then join them in code afterward. The QueryMultiple example does a similar thing with one database roundtrip.