📓 Many-to-Many Read Functionality

In the last lesson, we set up a many-to-many structure in our classes, created a join class, and configured and migrated our code into our database. Now we're ready to create controllers that will handle our new application structure. In this lesson, we'll focus on adding read functionality to the ItemsController, including the following:

Creating the TagsController with READ functionality for all tags (the Index() action) and an individual tag (the Details() action).
Adding a new navigation link on the homepage to access tags.
Creating the views for index and details.
Adding READ for join entities (viewing the tags that belong to each item and vice versa) in the following views:
- A category's detail page.
- An item's detail page.
- A tag's detail page.

READ: Creating the `TagsController` and `Index()` Action and View

Within the ToDoList production directory, create a new file called TagsController.cs within the Controllers directory and add the following code:

Controllers/TagsController.cs

using Microsoft.AspNetCore.Mvc;
using ToDoList.Models;
using System.Collections.Generic;
using System.Linq;
using Microsoft.EntityFrameworkCore;
using Microsoft.AspNetCore.Mvc.Rendering;

namespace ToDoList.Controllers
{
  public class TagsController : Controller
  {
    private readonly ToDoListContext _db;

    public TagsController(ToDoListContext db)
    {
      _db = db;
    }

    public ActionResult Index()
    {
      return View(_db.Tags.ToList());
    }
  }
}

The Index() route should look familiar to those we've created in the CategoriesController.cs and ItemsController.cs with one shortcut: instead of saving the list of tags to a variable and passing that into the View() method, we pass in the method call _db.Tags.ToList() directly as the argument to View().

Next, let's create the corresponding Index.cshtml. Within the Views directory, create another subdirectory called Tags and a file within called Index.cshtml with the following code:

Views/Tags/Index.cshtml

@{
  Layout = "_Layout";
}

@using ToDoList.Models;

<h1>Tags</h1>

@if (@Model.Count == 0)
{
  <h3>No tags have been added yet!</h3>
} 

@foreach (Tag tag in Model)
{
  <li>@Html.ActionLink($"{tag.Title}", "Details", new { id = tag.TagId })</li>
}

<p>@Html.ActionLink("Add new tag", "Create")</p>

<p>@Html.ActionLink("Home", "Index", "Home")</p>

Next, let's add a link to our tags from the homepage of our To Do List app. Open up ToDoList/Views/Home/Index.cshtml and add the following action link:

Views/Home/Index.cshtml

...

<p>@Html.ActionLink("See all tags", "Index", "Tags")</p>

READ: Creating the Tags `Details()` Action and View

Next, let's add the ability to see the details of a single tag. In the view, we'll list the title of the tag as well as every item that is currently associated with that tag. That means we're going to have to deliver the tag object to the Details.cshtml view, as well as the join entities the tag is associated with.

First, let's add the Details() action to the TagsController.cs:

Controllers/TagsController.cs

... 

    public ActionResult Details(int id)
    {
      Tag thisTag = _db.Tags
          .Include(tag => tag.JoinEntities)
          .ThenInclude(join => join.Item)
          .FirstOrDefault(tag => tag.TagId == id);
      return View(thisTag);
    }

...

Notice the new method we are using: ThenInclude(). Let's go over what's happening here.

Our _db.Tags expression gives us a list of Tag objects from the database. However, if we completed the query now (using the FirstOrDefault() method), we'd simply have an Tag without its related Items.

We need to .Include(tag => tag.JoinEntities) to load the JoinEntities property of each Tag. However, the JoinEntities property on an Tag is just a collection of join entities (List<ItemTag>), which are tracked by ids: ItemTagId, TagId, and ItemId. These are not the actual item objects related to a Tag.

We need the actual Item objects themselves, so we use ThenInclude() method to load the Item object associated with each ItemTag. Remember that an ItemTag is simply a reference to a relationship. Each ItemTag includes the id of a Tag as well as the id of an Item. With .ThenInclude(join => join.Item), we actually fetch the associated Item object for each ItemTag join entity.

Next, let's create the Tags/Details.cshtml view. Again, we'll make sure to display the Tag details as well as all of the Item objects associated with each Tag.

Views/Tags/Details.cshtml

@{
  Layout = "_Layout";
}

@using ToDoList.Models
@model ToDoList.Models.Tag

<h2>Tag Details</h2>
<hr />
<h3>@Html.DisplayNameFor(model => model.Title): @Html.DisplayFor(model => model.Title)</h3>

@if(@Model.JoinEntities.Count == 0)
{
  <p>This tag does not belong to any items.</p>
}
else
{
  <h4>Items the tag belongs to:</h4>
  <ul>
  @foreach(ItemTag join in Model.JoinEntities)
  {
    <li>@join.Item.Description</li>
  }
  </ul>
}

<p>@Html.ActionLink("Back to list", "Index")</p>

Most of this code should look familiar, so we'll point out a few important pieces:

We have an @using directive for ToDoList.Models so that we can reference the ItemTag class in our foreach loop.
We also have an @model directive for ToDoList.Models.Tag so that we can use the strongly typed HTML helper methods Html.DisplayNameFor() and @Html.DisplayFor.
Notice that we access @Model.JoinEntities.Count in our conditional: with this line of code, we are checking if the List<ItemTag> that we save to the Tag.JoinEntities property is empty, and if so, we deliver a message to the user stating that there are no items associated with the tag we're looking at.
If the List<ItemTag> is not empty, then we loop through the Tag.JoinEntities property and display each item's description: <li>@join.Item.Description</li>. A few notes:
- The variable join represents a single ItemTag join entity.
- To get the Item's description, we need to go through the ItemTag.Item property, which contains all of the Item object's data.

READ for Join Entities in the Item Details View

Next, we'll update the Details.cshtml views for both Categories and Items to display tags.

We'll start with updating Views/Items/Details.cshtml. Right now this view shows the description of the item along with the category it belongs to. After the update we make, the view will also include a list of tags that are associated with the item.

Here's the updated code:

Views/Items/Details.cshtml

@{
  Layout = "_Layout";
}

@using ToDoList.Models
@model ToDoList.Models.Item

<h2>Item Details</h2>
<hr />
<h3>@Html.DisplayNameFor(model => model.Description): @Html.DisplayFor(model => model.Description)</h3>  
<h3>@Html.DisplayNameFor(model => model.Category): @Html.DisplayFor(model => model.Category.Name)</h3>

@if(@Model.JoinEntities.Count == 0)
{
  <p>This item does not have any tags yet!</p>
}
else
{
  <h4>This item has the following tags:</h4>
  <ul>
    @foreach(ItemTag join in Model.JoinEntities)
    {
      <li>Tag: @join.Tag.Title</li>
    }
  </ul>
}

<p>@Html.ActionLink("Back to list", "Index")</p>
<p>@Html.ActionLink("Edit Item or Category", "Edit", new { id = Model.ItemId })</p>
<p>@Html.ActionLink("Delete Item", "Delete", new { id = Model.ItemId })</p>

The above addition looks very similar to how we display a list of items that belong to a tag. The main difference is in the naming. In summary, if the Item.JoinEntities.Count is equal to zero, then there are no tags that are associated with the item, so we deliver a message to the user about this. Otherwise, we loop through the Item.JoinEntities property, and for each join entity we access the Tag property to display the tag's title.

With our view ready to display join entities, we now need to update the Details() action in the ItemsController.cs to fetch the join entities and tags from the database when we get the data for the item. Here's the update we'll make:

Controllers/ItemsController.cs

... 

    public ActionResult Details(int id)
    {
      Item thisItem = _db.Items
          .Include(item => item.Category)
          .Include(item => item.JoinEntities)
          .ThenInclude(join => join.Tag)
          .FirstOrDefault(item => item.ItemId == id);
      return View(thisItem);
    }

...

What we've done is add a new Include() method to fetch the join entities, and a ThenInclude() method to fetch the actual tag object for each join entity.

Notice how we list an Include() method for each navigation property in the Item class: Item.Category and Item.JoinEntities. We can do this for as many navigation properties as we have and need to fetch.

As always, we end our database query with FirstOrDefault() if we want to fetch one object, or ToList() if we want to fetch a list of objects. There are many other methods we can use like OrderBy() or ToDictionary(). If you have not already done so, check out the MS Docs on the System.Linq.Enumerable class methods to learn about other methods we can use to query our database.

READ for Join Entities in the Category Details View

Next, we'll update our category details view to display not just the items that belong to each category, but also the tags that belong to each item. This is what the finished product will look like:

To Do List app's category details page that lists items and tags that belong to each item.

To make the above possible, we'll need a loop within a loop and some additional code to format our tags to display inline. Here's the updated code:

Views/Category/Details.cshtml

@{
  Layout = "_Layout";
}

@model ToDoList.Models.Category;
@using ToDoList.Models;
@using System.Collections.Generic;

<h2>Category Details</h2>
<hr />
<h3>@Html.DisplayNameFor(model => model.Name): @Html.DisplayFor(model => model.Name)</h3>

@if(@Model.Items.Count == 0)
{
  <p>This category does not contain any items</p>
}
else
{
  <h4>Items the category contains:</h4>
  <ul>
  @foreach(Item item in Model.Items)
  {
    string tags = "";
    @if(item.JoinEntities.Count == 0)
    {
      tags = "This item does not have any tags.";
    }
    else
    {
      List<string> tagList = new List<string>();
      @foreach(ItemTag join in item.JoinEntities)
      {
        tagList.Add(join.Tag.Title);
      }
      tags = String.Join(", ", tagList);
    }
    <li>@item.Description | Tags: @tags</li>
  }
  </ul>
}

<p>@Html.ActionLink("Back to categories", "Index")</p>
<p>@Html.ActionLink("Edit Category", "Edit", new { id = Model.CategoryId })</p>
<p>@Html.ActionLink("Delete Category", "Delete", new { id = Model.CategoryId })</p>
<p>@Html.ActionLink("Add new item", "Create", "Items")</p>

We'll focus on understanding the new code:

First notice that we've added a new @using directive for the System.Collections.Generic; namespace that allows use to use the List<T> type.

Next, notice how we've refactored the first (outer) foreach loop:

We've added branching logic to check whether there are any tags (join entities) associated with the item. If so, then we loop through the Item.JoinEntities property, and if not, then we display a message "This item does not have any tags.".
In order to display the list of tags inline next to an item, we make use of a variable called tags, and the String.Join() method:
- We use the variable tags to hold the display value for tags. It will be either a message saying there are no tags, or all of the tags associated with the item, separated by a comma.
- When the tags variable is set to the item's tags separated by a comma, we create this string by doing the following:
  - Creating an empty List<string> called tagList.
  - Looping through the join entities and adding each join entity's tag's title to the tagList.
  - Using String.Join() to join each list item in tagList into a string, separating each item with a comma and space.

Phew! That is a lot of new logic. Note that you can format your code however you like and you don't need to create complicated formatting in your own projects. You also don't need to display each item's tags on a category's detail page. However, you should consider what is best for a user's experience as far as navigating a site and accessing information. As always, have fun and try exploring something new.

With our category Details view ready to display each item's join entities, we now need to revisit our Details() action in the CategoriesController.cs to fetch not only a list of items, but each item's tags.

Here's the update we'll make:

Controllers/CategoriesController.cs

... 

    public ActionResult Details(int id)
    {
      Category thisCategory = _db.Categories
                                .Include(cat => cat.Items)
                                .ThenInclude(item => item.JoinEntities)
                                .ThenInclude(join => join.Tag)
                                .FirstOrDefault(category => category.CategoryId == id);
      return View(thisCategory);
    }

...

Category has only one navigation property, Category.Items; this is why there is only one Include() method call. If we want to access each item's tag(s), we need to use a series of ThenInclude() method calls to get the Item.JoinEntities data for each item, and then the JoinEntity.Tag tag data for each join entity.

We should now be able to run our application and navigate from the homepage to the tags index view. However, in order to view our new tag's details page (and the updates to our category and item details page), we'll have to first add some tags. Let's do that next.

READ: Creating the TagsController and Index() Action and View​

Adding Navigation for Tag Views from the Homepage​

READ: Creating the Tags Details() Action and View​

READ for Join Entities in the Item Details View​

READ for Join Entities in the Category Details View​

READ: Creating the `TagsController` and `Index()` Action and View

Adding Navigation for Tag Views from the Homepage

READ: Creating the Tags `Details()` Action and View

READ for Join Entities in the Item Details View

READ for Join Entities in the Category Details View