How do I get multi-mapping to work in Dapper? - ado.net

Question

I am trying to do the following with Dapper (and failing).
My POCOs (all code simplified) are:
public class Company
{
public int CompanyId { get; private set; }
public string CompanyName { get; private set; }
public Person CompanyAddress { get; private set; }
public Person Administrator { get; private set; }
}
public class Person
{
public int PersonId { get; private set; }
public string FirstName { get; private set; }
public string LastName { get; private set; }
}
In the database the Company table has a FK for CompanyAddress and Administrator which maps to the PersonID PK in the Person table. Based on this and this I think the follwoing is how I want to do this:
public static Company Select(IDbConnection connection, int id)
{
Trap.trap();
return connection.Query<Company, Person, Person, Company>("select * from Company left join Person address on Company.CompanyAddress = address.PersonId left join Person admin on Company.Administrator = admin.PersonId where Company.CompanyId = #Id",
(cmpy, addr, admin) => new { PersonId = id }).FirstOrDefault();
}
But that gives me a compile error on the "new { PersonId = id }". What am I getting wrong?

Answer 1

You need to provide the SplitOn parameter to specify where the next table/class starts. You also shouldn't create an anonymous type but use a new scope to initialize the Administrator property of Company:
string sql = #"select c.CompanyId,c.CompanyName, c.CompanyAddress,
address.PersonId, etc. ....
from Company c
left join Person address
on Company.CompanyAddress = address.PersonId
left join Person admin
on Company.Administrator = admin.PersonId
where Company.CompanyId = #Id";
string splitOn = "PersonId"; // maybe two parameters separated by comma, see comment below the answer
return connection.Query<Company, Person, Person, Company>(sql,
(Company cmpy, Person addr, Person admin) => { cmpy.Administrator = admin; return cmpy; }
,null,null,true,splitOn)
.FirstOrDefault();
However, i'm not sure if that works already since you have two joins to the same table. So i think you need an alias for all of the duplicate columns like PersonId. But this migt be helpful anyway.