如何使用 Dapper.net 映射同一张表中的多个对象？

Question

我有两个 SQL 表：Teams 和 Members。每个团队包含 3 名成员，在数据库中存储成员的 ID。

Sample Database

如何使用 Dapper.NET ORM 将成员对象映射到团队中？

public class Team
{
    public int? id { get; set; }
    public Member MemberA { get; set; }
    public Member MemberB { get; set; }
    public Member MemberC { get; set; }
}

public class Member
{
    public int? id { get; set; }
    public string Name { get; set; }
}



public IEnumerable<Team> GetTeams()
{
    string sql = "SELECT * FROM Teams t LEFT JOIN Members m ON t.MemberA=m.id AND t.MemberB=m.id AND t.MemberC=m.id";

    return m_connection.Query<Team, Member, Member, Member, Team>(sql, (t, m1, m2, m3) =>
    {
        t.MemberA = m1;
        t.MemberB = m2;
        t.MemberC = m3;

        return t;
    }, splitOn: "MemberA,MemberB,MemberC");
}

Answer 1

您需要修复您的 sql 查询以与成员表正确连接。

改成

string sql = @"SELECT t.ID, t.MemberA, m1.Id, m1.Name, 
                            t.MemberB, m2.Id, m2.Name, 
                            t.MemberC, m3.Id, m3.Name
               FROM Teams t LEFT JOIN Members m1 ON t.MemberA=m1.id 
                            LEFT JOIN Members m2 ON t.MemberB=m2.id
                            LEFT JOIN Members m3 ON t.MemberC=m3.id";

您的简洁代码将按照您预期的方式工作，填充检索到的每个 Team 的三个 Member 实例。

请注意，当您使用多重映射时，您需要将 SplitOn 元素放置在适当的位置，以便 Dapper 了解您创建三个不同成员变量的要求。

MS-Access 版本

string sql = @"SELECT t.ID, t.MemberA, m1.Id, m1.[Name], 
                            t.MemberB, m2.Id, m2.[Name], 
                            t.MemberC, m3.Id, m3.[Name]
               FROM (((Teams t LEFT JOIN Members m1 ON t.MemberA=m1.id) 
                            LEFT JOIN Members m2 ON t.MemberB=m2.id)
                            LEFT JOIN Members m3 ON t.MemberC=m3.id)";