3

我有一个这样的实体：

public class Employment
{
    public virtual Company Company {get; set;}
    public virtual Person Person {get; set;}
    public virtual string Description {get; set;}
}

提供两个其他实体之间的关系。他们有相应的 DTO，我想返回一个包含有关人员和公司的所有信息的结果集。查询是在就业表上执行的，我的问题是 Hibernate 为每个公司和个人生成一个选择语句。

在我的数据库中，Employment 表有 1000 行。当我将它们映射到 DTO 时，Nhibernate 生成 2001 个选择语句，一个用于就业列表，一个用于每个人和公司的选择。

我希望hibernate一次获取所有信息，在SQL中我会做这样的事情：

SELECT e.Description, c.A, c.B, c.C, p.D, p.E, p.F
FROM Employment e
JOIN Company c ON e.Company_Id = c.Company_Id
JOIN Person p ON e.Person_Id = p.Person_Id;

甚至

SELECT Description FROM Employment;

SELECT c.A, c.B, c.C FROM Employment e
JOIN Company c ON e.Company_Id = c.Company_Id;

SELECT p.D, p.E, p.F FROM Employment e
JOIN Person p ON e.Person_Id = p.Person_Id;

I am a pretty fresh user of nHibernate, QueryOver.I welcome Linq-To-Entities answers as well but I prefer to avoid LINQ query expressions.

I've looked all over the web, read about JoinQuery, JoinAlias and Fetch and have come as far as something like this:

//This works, but the objects are retrieved as PersonProxy and CompanyProxy,
//generating 2 SELECT statements for each Employment I map to EmploymentDto
var queryOver =
    session.QueryOver<Employment>()
    .Fetch(x => x.Person).Eager
    .Fetch(x => x.Company).Eager
var mapResult = MappingEngine.Map<IList<EmploymentDto>>(queryOver.List());    


//This works, but the objects are still retrieved as PersonProxy and CompanyProxy,
var queryOver =
    session.QueryOver<Employment>()
        .JoinAlias(x => x.Person, () => personAlias, JoinType.InnerJoin)
        .JoinAlias(x => x.Company, () => companyAlias, JoinType.InnerJoin);
var mapResult = MappingEngine.Map<IList<EmploymentDto>>(queryOver.List());

JoinQuery provided the same result aswell. I feel I am missing something important here. Something should be done in the query or before .List() to fetch all child entities instead of loading a list with lots of Employment entities loaded with PersonProxy and CompanyProxy. I can, however, not find out how...

EDIT: Added mapping

Database tables:

TABLE Company(
Id,
A,
B,
C)

TABLE Person(
Id,
D,
E,
F);

TABLE Employment(
Person_Id,
Company_Id,
Description);

Entities

public class Company
{
    public virtual string Id { get; set; }
    public virtual string A { get; set; }
    public virtual bool B { get; set; }
    public virtual bool C { get; set; }
}

public class Person
{
    public virtual string Id { get; set; }
    public virtual string D { get; set; }
    public virtual string E { get; set; }
    public virtual string F { get; set; }
}

public class Employment
{
    public virtual Person Person { get; set; }
    public virtual Company Company { get; set; }
    public virtual string Description { get; set; }

    public override bool Equals(object obj)
    {
        Employment toCompare = obj as Employment;
        if (toCompare == null)
            return false;
        return (this.GetHashCode() != toCompare.GetHashCode());
    }

    public override int GetHashCode()
    {
        unchecked
        {
            int results = Person != null ? Person.GetHashCode() : 0;
            results = (results * 397) ^ (Company != null ? Company.GetHashCode() : 0);
            results = (results * 397) ^ (Description != null ? Description.GetHashCode() : 0);
            return results;
        }
    }
}

Mapping

public class CompanyMap : SyncableClassMap<Company>
{
    public CompanyMap()
    {
        Table("Company");
        Id(x => x.Id).Column("Id").GeneratedBy.Assigned();
        Map(x => x.A).Column("A");
        Map(x => x.B).Column("B").CustomType<YesNoType>();
        Map(x => x.C).Column("C").CustomType<YesNoType>();
    }
}

public class PersonMap : SyncableClassMap<Person>
{
    public PersonMap()
    {
        Table("Person");
        Id(x => x.Id).Column("Id").GeneratedBy.Assigned();
        Map(x => x.D).Column("D");
        Map(x => x.E).Column("E");
        Map(x => x.F).Column("F");
    }
}

public class EmploymentMap : ClassMap<Employment>
{
    public EmploymentMap()
    {
        Table("Employment");
        CompositeId()
            .KeyReference(x => x.Person, "Person_Id")
            .KeyReference(x => x.Company, "Company_Id");
        Map(x => x.Description, "Description");
    }
}
4

2 回答 2

4

after your edit i see you have a keyreference instead of a normal many-to-one.

Unfortunatly this seems to be a limitation of QueryOver/Criteria which does not eager load keyreferences even with Fetchmode specified. However Linq to NH does not have this limitation. Change the query to

using NHibernate.Linq;

var results = session.Query<Employment>()
    .Fetch(x => x.Person)
    .Fetch(x => x.Company)
    .ToList();
于 2012-05-23T05:44:41.373 回答
0

I have experienced the same problem you're describing here. I'll take your last code snippet as an example, since that's the way I've made it work:

//This works, but the objects are still retrieved as PersonProxy and CompanyProxy,
var queryOver =
    session.QueryOver<Employment>()
        .JoinAlias(x => x.Person, () => personAlias, JoinType.InnerJoin)
        .JoinAlias(x => x.Company, () => companyAlias, JoinType.InnerJoin);
var mapResult = MappingEngine.Map<IList<EmploymentDto>>(queryOver.List());

First of all, you shouldn't have to specify JoinType.InnerJoin since that's the default join type. Same as with you, I've also found that the persons and companies are loaded lazily this way.

However, if you change the join type to JoinType.LeftOuterJoin, you'll see that everything is loaded eagerly. At least that's what I've experienced. So try change your code to the following:

//This works, but the objects are still retrieved as PersonProxy and CompanyProxy,
var queryOver =
    session.QueryOver<Employment>()
        .JoinAlias(x => x.Person, () => personAlias, JoinType.LeftOuterJoin)
        .JoinAlias(x => x.Company, () => companyAlias, JoinType.LeftOuterJoin);
var mapResult = MappingEngine.Map<IList<EmploymentDto>>(queryOver.List());

I can't explain to you why it is this way, I've just found this to work out of own experiences. And if it's problematic for you doing a left outer join, you can instead try to filter accordingly in your code before (or while) doing the mapping.

于 2012-05-15T06:10:01.583 回答