many-to-many relationship in mvc and EF

Question

I have created a blog site where users can Post a blog, in my model and database using Entity Framework there is a many-to-many relationship between Posts and Tags.

In my database I also have PostTagMap, however I don't see this in my EF Model. Not sure if I need to. The PostTagMap just contains the unique identifier for both Post and Tag nothing else.

(should be noted I created the database from the model)

On my Create Post View (and controller) I want the users to have the option to create tags (along with there post) making sure that they are identified with one another. However I cant find a way to do this.

Post Model:

public partial class Post
{
    public Post()
    {
        this.Tags = new HashSet<Tag>();
    }

    public int Id { get; set; }
    public string BlogUserEmail { get; set; }
    public int CategoryId { get; set; }
    public string Title { get; set; }
    public string ShortDescription { get; set; }

    [AllowHtml]
    public string Description { get; set; }
    public string Meta { get; set; }
    public string UrlSlug { get; set; }
    public bool Published { get; set; }
    public System.DateTime PostedOn { get; set; }
    public Nullable<System.DateTime> Modified { get; set; }

    public virtual BlogUser BlogUser { get; set; }
    public virtual Category Category { get; set; }
    public virtual ICollection<Tag> Tags { get; set; }
}


<h2>Create</h2>

@using (Html.BeginForm()) {
    @Html.AntiForgeryToken()
    @Html.ValidationSummary(true)

    <fieldset>
        <legend>Post</legend>

        <div class="editor-label">
            @Html.HiddenFor(model => model.BlogUserEmail, User.Identity.Name)
        </div>
        <div class="editor-field">
            @Html.ValidationMessageFor(model => model.BlogUserEmail)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.CategoryId, "Category")
        </div>
        <div class="editor-field">
            @Html.DropDownList("CategoryId", String.Empty)
            @Html.ValidationMessageFor(model => model.CategoryId)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.Title)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.Title)
            @Html.ValidationMessageFor(model => model.Title)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.ShortDescription)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.ShortDescription)
            @Html.ValidationMessageFor(model => model.ShortDescription)
        </div>

        <div class="editor-label">
            @Html.TextAreaFor(model => model.Description)
        </div>
        <div class="editor-field">
            @Html.ValidationMessageFor(model => model.Description)
        </div>

        <p>
            <input type="submit" value="Create" />
        </p>
    </fieldset>

Edit:

There is no way for me to use a EditorFor field in my Create View for tags. This is because the Create view is bound to @Model MyBlogger.Post.

Tags only show up as a collection and as such if I say:

@Html.EditorFor(model => m.Tags) 

Then it shows blank in my view when rendered.

I have tried a foreach however that did not work. For Instance trying this in my Create view:

@foreach (var item in Model.Tags)
{
    @Html.EditorFor(model => item.Name)
    @Html.EditorFor(model => item.Description)
    @Html.EditorFor(model => item.UrlSlug)
}

I get the error: Object reference not set to an instance of an object.

Answer 1

Many to many tables are not displayed in the EF model. For adding rows to it you have to make something like: PostObject1.Tags.Add(TagObject1) or TagObject1.Posts.Add(PostObject1).

Here is how to do it: Insert/Update Many to Many Entity Framework . How do I do it?

For selecting rows you need the EF object and access to the list of the other table.

Answer 2

In a Many-to-Many relationship if you are using DataAnnotation EF will create the Many-to-Many relationship. in your case

Post class has a collection navigation property for Tag

public virtual ICollection<Tag> Tags { get; set; }

And Tag has a collection navigation property for Post

 public virtual ICollection<Post> Posts { get; set; }

Which will create a Many-to-Many relationship between Post and Tag.

---

In your view:

There are couple of ways to add/get the value of the Tag ID , one way to bind it to the model Post is to try the following , since you already have a relationship between the two models:

      <div class="editor-field">
            @Html.EditorFor(model => model.Tags.Id)
            @Html.ValidationMessageFor(model => model.Tags.Id)
        </div>

model.Tags returns a collection of Tags if you need an attribute you need to specify it explicitly (such as Id, Name ...)

Insert in M-to-M relationship:

if (ModelState.IsValid)
        {
            Tag tag = new tag{id=post.Tags.Id};
            db.Tag.Attach(tag);
            db.Tag.Add(tag);
            db.Posts.Add(post);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

Answer 3

Associated(Mapping) tables may not show up in the EF Model, but you can easily add new tag

post.Tags.add(new Tag(){ Name = ""});

As a good practice You should create View Models instead of using the Models directly for views,

here is the link to help you with that Link

For Tags you can either do EditorTemplate(Recommended) or For loop

Here is the for loop example

@for(var i=0;i<Model.Tags.Count();i++) { @Html.EditorFor(model => model.Tags[i].Name) }

For Create New

Just have Add New button, and onclick, just add a new text box, but set the Id to Tags[i].Name, where i is the Count()+1 of the tags.