问题
In .NET Entity Framework, what is the best way to have a (custom) join table with extra attributes (other than ids) and/or associate this join table with others via separate model? In Ruby on Rails we can have a model for the join table, like:
Item.rb (model)
:has_many => :buyers, :through=>:invoice
...
Buyers.rb (model)
:has_many => :items, :through=>:invoice
...
Invoice.rb (model)
:belongs_to :item
:belongs_to :buyer
....
Then we can use: Item.first.buyers, Buyers.first.items and Buyer.create(:items=>Item.create(:name=>'random')) etc. just like when we use automated join table without model (using has_and_belongs_to_many).
In Visual Studio 2010's "Add Association" dialog, if we select multiplicity as *(Many) there is no option to select a join table (with model). Is there a way to do it manually?
回答1:
Yes, you can get something pretty close. I'm not quite sure how to set this up in the designer since I only work with codefirst.
Here's an example:
Student -> StudentFloor <- Floor
public class Student
{
public int Id { get; set; }
// ... properties ...
// Navigation property to your link table
public virtual ICollection<StudentFloor> StudentFloors { get; set; }
// If you wanted to have a property direct to the floors, just add this:
public IEnumerable<Floor> Floors
{
get
{
return StudentFloors.Select(ft => ft.Floor);
}
}
}
The linking table:
public class StudentFloor
{
#region Composite Keys
// Be sure to set the column order and key attributes.
// Convention will link them to the navigation properties
// below. The database table will be created with a
// compound key.
[Key, Column(Order = 0)]
public int StudentId { get; set; }
[Key, Column(Order = 1)]
public int FloorId { get; set; }
#endregion
// Here's the custom data stored in the link table
[Required, StringLength(30)]
public string Room { get; set; }
[Required]
public DateTime Checkin { get; set; }
// Navigation properties to the outer tables
[Required]
public virtual Student Student { get; set; }
[Required]
public virtual Floor Floor { get; set; }
}
Finally, the other side of the many-to-many:
public class Floor
{
public int Id { get; set; }
// ... Other properties.
public virtual ICollection<StudentFloor> StudentFloors { get; set; }
}
回答2:
UPDATE to Leniency's answer:
We can also create two one-to-many relationships with Model first approach. Either way we cannot have model binding as happen in pure M2M relationship (without payload or pure join tables - PJT).
Also, in (scaffold) controller, we can use a view model for the CRUD operations as per the requirement. Supposedly, we have a FloorViewModel with the following definition:
public class FloorViewModel
{
private Model2Container context = new Model2Container();
[Display(Name = "Student List")]
[Required(ErrorMessage = "Please select atleast one student from the list.")]
public int[] MyStudents { get; set; }
public Floor MyFloor { get; set; }
public MultiSelectList StudentsList { get; set; }
public StudentFloorJoin Join { get; set; }
}
The create action in controller would be:
//
// GET: /Floor/Create
public ActionResult Create()
{
var model = new FloorViewModel() { StudentsList = new MultiSelectList(context.Students, "Id", "Name") };
return View(model);
}
//
// POST: /Floor/Create
[HttpPost]
public ActionResult Create(FloorViewModel floor)
{
if (ModelState.IsValid)
{
context.Floors.Add(floor.MyFloor);
context.SaveChanges();
}
foreach (var student in floor.MyStudents)
{
context.StudentFloorJoins.Add(new StudentFloorJoin() { Student = context.Students.Find(student), Floor = floor.MyFloor, Room = floor.Join.Room });
}
if (ModelState.IsValid)
{
context.SaveChanges();
return RedirectToAction("Index");
}
context.Floors.Remove(floor.MyFloor);
floor.StudentsList = new MultiSelectList(context.Students, "Id", "Name", floor.MyStudents);
return View(floor);
}
and the Edit action would be something like:
//
// GET: /Floor/Edit
public ActionResult Edit(int id)
{
Floor floor = context.Floors.Single(x => x.Id == id);
int[] ids = (from i in floor.StudentFloorJoins select i.Student.Id).ToArray();
var model = new FloorViewModel() { StudentsList = new MultiSelectList(context.Students, "Id", "Name", ids), MyFloor = floor, Join = new StudentFloorJoin() { Room = floor.StudentFloorJoins.Count == 0 ? "" : floor.StudentFloorJoins.First().Room } };
return View(model);
}
//
// POST: /Floor/Edit
[HttpPost]
public ActionResult Edit(FloorViewModel floor)
{
if (ModelState.IsValid)
{
var item = floor.MyFloor;
var itemEntry1 = context.Entry<Floor>(item);
itemEntry1.State = EntityState.Modified;
var query = (from i in context.StudentFloorJoins where i.Floor.Id == item.Id select i.Id);
foreach (var studentfloor in query)
{
context.StudentFloorJoins.Remove(context.StudentFloorJoins.Find(studentfloor));
}
context.SaveChanges();
foreach (var student in floor.MyStudents)
{
context.StudentFloorJoins.Add(new StudentFloorJoin() { Student = context.Students.Find(student), Floor = floor.MyFloor, Room = floor.Join.Room });
}
context.SaveChanges();
return RedirectToAction("Index");
}
floor.StudentsList = new MultiSelectList(context.Students, "Id", "Name", floor.MyStudents);
return View(floor);
}
In View, we can send the FloorModelView's object like:
@model ManyToManyAutoGen.Models.FloorViewModel
@{
ViewBag.Title = "Create";
}
<h2>Create</h2>
<script src="@Url.Content("~/Scripts/jquery.validate.min.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.min.js")" type="text/javascript"></script>
@using (Html.BeginForm()) {
@Html.ValidationSummary(true)
<fieldset>
<legend>Floor</legend>
@Html.Partial("_CreateOrEdit", Model)
<p>
<input type="submit" value="Create" />
</p>
</fieldset>
}
<div>
@Html.ActionLink("Back to List", "Index")
</div>
and finally, _CreateOrEdit partial looks like:
@model ManyToManyAutoGen.Models.FloorViewModel
@* This partial view defines form fields that will appear when creating and editing entities *@
<div class="editor-label">
@Html.LabelFor(model => model.MyFloor.FloorName)
</div>
<div class="editor-field">
@Html.EditorFor(model => model.MyFloor.FloorName)
@Html.ValidationMessageFor(model => model.MyFloor.FloorName)
</div>
<div class="editor-label">
@Html.LabelFor(model => model.MyStudents)
</div>
<div class="editor-field">
@Html.ListBoxFor(model => model.MyStudents, Model.StudentsList)
@Html.ValidationMessageFor(model => model.MyStudents)
</div>
<div class="editor-label">
@Html.LabelFor(model => model.Join.First().Room)
</div>
<div class="editor-field">
@Html.EditorFor(model => model.Join.First().Room)
@Html.ValidationMessageFor(model => model.Join)
</div>
来源：https://stackoverflow.com/questions/8537356/rails-has-many-through-equivalent-in-asp-net-mvc3