Add a model to an ASP.NET Core MVC app

By Rick Anderson and Tom Dykstra

In this section, you add classes for managing movies in a database. These classes will be the "Model" part of the MVC app.

You use these classes with Entity Framework Core (EF Core) to work with a database. EF Core is an object-relational mapping (ORM) framework that simplifies the data access code that you have to write.

The model classes you create are known as POCO classes (from Plain Old CLR Objects) because they don't have any dependency on EF Core. They just define the properties of the data that will be stored in the database.

In this tutorial, you write the model classes first, and EF Core creates the database. An alternate approach not covered here is to generate model classes from an existing database. For information about that approach, see ASP.NET Core - Existing Database.

Add a data model class

Right-click the Models folder > Add > Class. Name the class Movie.

Add the following properties to the Movie class:

using System;
using System.ComponentModel.DataAnnotations;

namespace MvcMovie.Models
{
    public class Movie
    {
        public int Id { get; set; }
        public string Title { get; set; }

        [DataType(DataType.Date)]
        public DateTime ReleaseDate { get; set; }
        public string Genre { get; set; }
        public decimal Price { get; set; }
    }
}

The Movie class contains:

  • The Id field which is required by the database for the primary key.

  • [DataType(DataType.Date)]: The DataType attribute specifies the type of the data (Date). With this attribute:

    • The user is not required to enter time information in the date field.
    • Only the date is displayed, not time information.

DataAnnotations are covered in a later tutorial.

Scaffold the movie model

In this section, the movie model is scaffolded. That is, the scaffolding tool produces pages for Create, Read, Update, and Delete (CRUD) operations for the movie model.

In Solution Explorer, right-click the Controllers folder > Add > New Scaffolded Item.

view of above step

In the Add Scaffold dialog, select MVC Controller with views, using Entity Framework > Add.

Add Scaffold dialog

Complete the Add Controller dialog:

  • Model class: Movie (MvcMovie.Models)
  • Data context class: Select the + icon and add the default MvcMovie.Models.MvcMovieContext

Add Data context

  • Views: Keep the default of each option checked
  • Controller name: Keep the default MoviesController
  • Select Add

Add Controller dialog

Visual Studio creates:

  • An Entity Framework Core database context class (Data/MvcMovieContext.cs)
  • A movies controller (Controllers/MoviesController.cs)
  • Razor view files for Create, Delete, Details, Edit, and Index pages (Views/Movies/*.cshtml)

The automatic creation of the database context and CRUD (create, read, update, and delete) action methods and views is known as scaffolding.

If you run the app and click on the Mvc Movie link, you get an error similar to the following:

An unhandled exception occurred while processing the request.

SqlException: Cannot open database "MvcMovieContext-<GUID removed>" requested by the login. The login failed.
Login failed for user 'Rick'.

System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString

You need to create the database, and you use the EF Core Migrations feature to do that. Migrations lets you create a database that matches your data model and update the database schema when your data model changes.

Initial migration

In this section, the Package Manager Console (PMC) is used to:

  • Add an initial migration.
  • Update the database with the initial migration.

From the Tools menu, select NuGet Package Manager > Package Manager Console.

PMC menu

In the PMC, enter the following commands:

Add-Migration Initial
Update-Database

The Add-Migration command generates code to create the initial database schema.

The preceding commands generate the following warning: "No type was specified for the decimal column 'Price' on entity type 'Movie'. This will cause values to be silently truncated if they do not fit in the default precision and scale. Explicitly specify the SQL server column type that can accommodate all the values using 'HasColumnType()'."

You can ignore that warning, it will be fixed in a later tutorial.

The schema is based on the model specified in the DbContext (In the Models/MvcMovieContext.cs file). The InitialCreate argument is used to name the migrations. Any name can be used, but by convention a name is selected that describes the migration.

The ef database update command runs the Up method in the Migrations/<time-stamp>_InitialCreate.cs file. The Up method creates the database.

Examine the context registered with dependency injection

ASP.NET Core is built with dependency injection. Services (such as the EF Core DB context) are registered with dependency injection during application startup. Components that require these services (such as Razor Pages) are provided these services via constructor parameters. The constructor code that gets a DB context instance is shown later in the tutorial.

The scaffolding tool automatically created a DB context and registered it with the dependency injection container.

Examine the Startup.ConfigureServices method. The highlighted line was added by the scaffolder:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        // This lambda determines whether user consent for non-essential cookies 
        // is needed for a given request.
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });


    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

    services.AddDbContext<MvcMovieContext>(options =>
         options.UseSqlServer(Configuration.GetConnectionString("MvcMovieContext")));
}

The MvcMovieContext coordinates EF Core functionality (Create, Read, Update, Delete, etc.) for the Movie model. The data context (MvcMovieContext) is derived from Microsoft.EntityFrameworkCore.DbContext. The data context specifies which entities are included in the data model:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;

namespace MvcMovie.Models
{
    public class MvcMovieContext : DbContext
    {
        public MvcMovieContext (DbContextOptions<MvcMovieContext> options)
            : base(options)
        {
        }

        public DbSet<MvcMovie.Models.Movie> Movie { get; set; }
    }
}

The preceding code creates a DbSet<Movie> property for the entity set. In Entity Framework terminology, an entity set typically corresponds to a database table. An entity corresponds to a row in the table.

The name of the connection string is passed in to the context by calling a method on a DbContextOptions object. For local development, the ASP.NET Core configuration system reads the connection string from the appsettings.json file.

The schema is based on the model specified in the MvcMovieContext (In the Data/MvcMovieContext.cs file). The Initial argument is used to name the migrations. Any name can be used, but by convention a name that describes the migration is used. See Introduction to migrations for more information.

The Update-Database command runs the Up method in the Migrations/{time-stamp}_InitialCreate.cs file, which creates the database.

Test the app

  • Run the app and append /Movies to the URL in the browser (http://localhost:port/movies).

If you get a database exception similar to the following:

SqlException: Cannot open database "MvcMovieContext-GUID" requested by the login. The login failed.
Login failed for user 'User-name'.

You missed the migrations step.

  • Test the Create link.

    Note

    You may not be able to enter decimal commas in the Price field. To support jQuery validation for non-English locales that use a comma (",") for a decimal point and for non US-English date formats, the app must be globalized. For globalization instructions, see this GitHub issue.

  • Test the Edit, Details, and Delete links.

Examine the Startup class:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        // This lambda determines whether user consent for non-essential cookies 
        // is needed for a given request.
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });


    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

    services.AddDbContext<MvcMovieContext>(options =>
         options.UseSqlServer(Configuration.GetConnectionString("MvcMovieContext")));
}

The preceding highlighted code shows the movie database context being added to the Dependency Injection container:

  • services.AddDbContext<MvcMovieContext>(options => specifies the database to use and the connection string.
  • => is a lambda operator

Open the Controllers/MoviesController.cs file and examine the constructor:

public class MoviesController : Controller
{
    private readonly MvcMovieContext _context;

    public MoviesController(MvcMovieContext context)
    {
        _context = context;
    }

The constructor uses Dependency Injection to inject the database context (MvcMovieContext) into the controller. The database context is used in each of the CRUD methods in the controller.

Strongly typed models and the @model keyword

Earlier in this tutorial, you saw how a controller can pass data or objects to a view using the ViewData dictionary. The ViewData dictionary is a dynamic object that provides a convenient late-bound way to pass information to a view.

MVC also provides the ability to pass strongly typed model objects to a view. This strongly typed approach enables better compile time checking of your code. The scaffolding mechanism used this approach (that is, passing a strongly typed model) with the MoviesController class and views when it created the methods and views.

Examine the generated Details method in the Controllers/MoviesController.cs file:

// GET: Movies/Details/5
public async Task<IActionResult> Details(int? id)
{
    if (id == null)
    {
        return NotFound();
    }

    var movie = await _context.Movie
        .FirstOrDefaultAsync(m => m.Id == id);
    if (movie == null)
    {
        return NotFound();
    }

    return View(movie);
}

The id parameter is generally passed as route data. For example https://localhost:5001/movies/details/1 sets:

  • The controller to the movies controller (the first URL segment).
  • The action to details (the second URL segment).
  • The id to 1 (the last URL segment).

You can also pass in the id with a query string as follows:

https://localhost:5001/movies/details?id=1

The id parameter is defined as a nullable type (int?) in case an ID value isn't provided.

A lambda expression is passed in to FirstOrDefaultAsync to select movie entities that match the route data or query string value.

var movie = await _context.Movie
    .FirstOrDefaultAsync(m => m.Id == id);

If a movie is found, an instance of the Movie model is passed to the Details view:

return View(movie);

Examine the contents of the Views/Movies/Details.cshtml file:

@model MvcMovie.Models.Movie

@{
    ViewData["Title"] = "Details";
}

<h1>Details</h1>

<div>
    <h4>Movie</h4>
    <hr />
    <dl class="row">
        <dt class="col-sm-2">
            @Html.DisplayNameFor(model => model.Title)
        </dt>
        <dd class="col-sm-10">
            @Html.DisplayFor(model => model.Title)
        </dd>
        <dt class="col-sm-2">
            @Html.DisplayNameFor(model => model.ReleaseDate)
        </dt>
        <dd class="col-sm-10">
            @Html.DisplayFor(model => model.ReleaseDate)
        </dd>
        <dt class="col-sm-2">
            @Html.DisplayNameFor(model => model.Genre)
        </dt>
        <dd class="col-sm-10">
            @Html.DisplayFor(model => model.Genre)
        </dd>
        <dt class="col-sm-2">
            @Html.DisplayNameFor(model => model.Price)
        </dt>
        <dd class="col-sm-10">
            @Html.DisplayFor(model => model.Price)
        </dd>
    </dl>
</div>
<div>
    <a asp-action="Edit" asp-route-id="@Model.Id">Edit</a> |
    <a asp-action="Index">Back to List</a>
</div>

By including a @model statement at the top of the view file, you can specify the type of object that the view expects. When you created the movie controller, the following @model statement was automatically included at the top of the Details.cshtml file:

@model MvcMovie.Models.Movie

This @model directive allows you to access the movie that the controller passed to the view by using a Model object that's strongly typed. For example, in the Details.cshtml view, the code passes each movie field to the DisplayNameFor and DisplayFor HTML Helpers with the strongly typed Model object. The Create and Edit methods and views also pass a Movie model object.

Examine the Index.cshtml view and the Index method in the Movies controller. Notice how the code creates a List object when it calls the View method. The code passes this Movies list from the Index action method to the view:

// GET: Movies
public async Task<IActionResult> Index()
{
    return View(await _context.Movie.ToListAsync());
}

When you created the movies controller, scaffolding automatically included the following @model statement at the top of the Index.cshtml file:

@model IEnumerable<MvcMovie.Models.Movie>

The @model directive allows you to access the list of movies that the controller passed to the view by using a Model object that's strongly typed. For example, in the Index.cshtml view, the code loops through the movies with a foreach statement over the strongly typed Model object:

@model IEnumerable<MvcMovie.Models.Movie>

@{
    ViewData["Title"] = "Index";
}

<h1>Index</h1>

<p>
    <a asp-action="Create">Create New</a>
</p>
<table class="table">
    <thead>
        <tr>
            <th>
                @Html.DisplayNameFor(model => model.Title)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.ReleaseDate)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Genre)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Price)
            </th>
            <th></th>
        </tr>
    </thead>
    <tbody>
@foreach (var item in Model) {
        <tr>
            <td>
                @Html.DisplayFor(modelItem => item.Title)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.ReleaseDate)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Genre)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Price)
            </td>
            <td>
                <a asp-action="Edit" asp-route-id="@item.Id">Edit</a> |
                <a asp-action="Details" asp-route-id="@item.Id">Details</a> |
                <a asp-action="Delete" asp-route-id="@item.Id">Delete</a>
            </td>
        </tr>
}
    </tbody>
</table>

Because the Model object is strongly typed (as an IEnumerable<Movie> object), each item in the loop is typed as Movie. Among other benefits, this means that you get compile time checking of the code:

Additional resources