Nhibernate查询

Question

I have this domain model,

class UserInfo{
    int UserId{get;set;}
    string UserName{get;set;}
    IList<Task> Tasks{get;set;}
}

class Task{
    int TaskId{get;set;}
    string TaskName{get;set;}
    UserInfo AssignedTo{get;set;}
}

class UserMap: ClassMap<UserInfo>
{ 
    public UserMap()
    {
        Id(x=>x.UserId);
        Map(x=>x.UserName)
        HasMany(x=>x.Tasks);
    }
 }

Now, a user may have any number of tasks assigned to him, but Active is always none or one. This is stored in another table and defined through another entity. So, if there is an active task for a child, there exists a record in ActiveTask table.

class ActiveTask{
    int Id{get;set;}
    int UserId{get;set;}
    int TaskId{get;set;}
    datetime CreatedOn{get;set;}
}

Questions,

• How to map ACTIVETASK with USERINFO
• I want to fetch a list of users, along with active task (if any).

Please suggest how to do it

Answer 1

I think you'd be hard-pressed to find the one good answer because it will depend on other factors such as the number of tasks you expect a given user to have. That said, it sounds like you're trying to use a single collection of Task and simply mark one as active. From a domain perspective, I'd bet this is the reality of things. People have many tasks, one is active. An active task is still a task.

If that's the case, then one option is to add an IsActive property to your Task object, then set your fetch strategy in the mapping to be eager (otherwise you'll have a select n+1 issue). Then, your ActiveTask property could use linq to select the Task in the collection that has Active == true .

If you went this route, then you'd want to add logic in your class to set a task active such that all other tasks were set to inactive. Something like this:

class UserInfo
{
    public virtual int UserId{get;set;}
    public virtual string UserName{get;set;}
    public virtual IList<Task> Tasks{get;set;}

    public virtual Task ActiveTask
    {
        get
        {
            return Tasks.FirstOrDefault(t => t.IsActive == true);
        {
    }

    public void SetActive(Task task)
    {
        // Set all tasks to inactive
        // Set the task in question to active
    }
}

If it were me I would probably also change the Tasks collection from an autoproperty to having a backing field and use protected internal for setting so you could ensure that the list is only assigned during mapping.

Note that if you have a large number of users or tasks the eager fetch strategy could become costly. In this case, then you probably would want to use a separate table to track which task is active and map that way.

Update

I went back and checked on some other projects I had done that had similar requirements, and in one case I actually changed it so the parent object held a reference to the active child object. So your UserInfo table in the database would have an FK column of ActiveTask_Id which would relate to the Task table's PK. Then your mapping of the user would be like (assuming you're overriding an automap):

public class UserInfoOverride : IAutoMappingOverride<UserInfo>
{
    public void Override(AutoMapping<UserInfo> mapping)
    {
        mapping.References(x => x.ActiveTask).Fetch.Join();
    }
}

You would then simply set a task to the ActiveTask property of your UserInfo object and save.