Part 8, add a new field to an ASP.NET Core MVC app

By Rick Anderson

In this section Entity Framework Code First Migrations is used to:

• Add a new field to the model.
• Migrate the new field to the database.

When EF Code First is used to automatically create a database, Code First:

• Adds a table to the database to track the schema of the database.
• Verifies the database is in sync with the model classes it was generated from. If they aren't in sync, EF throws an exception. This makes it easier to find inconsistent database/code issues.

Add a Rating Property to the Movie Model

Add a Rating property to Models/Movie.cs:

using System;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

namespace MvcMovie.Models;

public class Movie
{
    public int Id { get; set; }
    public string? Title { get; set; }

    [Display(Name = "Release Date")]
    [DataType(DataType.Date)]
    public DateTime ReleaseDate { get; set; }
    public string? Genre { get; set; }
    
    [Column(TypeName = "decimal(18, 2)")]
    public decimal Price { get; set; }
    public string? Rating {  get; set; }
}

Build the app

Visual Studio

Press Ctrl+Shift+B

Visual Studio Code

From the View menu, select Terminal and enter the following command:

dotnet build

Visual Studio for Mac

Press ⌘+B

Because you've added a new field to the Movie class, you need to update the property binding list so this new property will be included. In MoviesController.cs, update the [Bind] attribute for both the Create and Edit action methods to include the Rating property:

[Bind("Id,Title,ReleaseDate,Genre,Price,Rating")]

Update the view templates in order to display, create, and edit the new Rating property in the browser view.

Edit the /Views/Movies/Index.cshtml file and add a Rating field:

<table class="table">
    <thead>
        <tr>
            <th>
                @Html.DisplayNameFor(model => model.Movies![0].Title)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Movies![0].ReleaseDate)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Movies![0].Genre)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Movies![0].Price)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Movies![0].Rating)
            </th>
            <th></th>
        </tr>
    </thead>
    <tbody>
        @foreach (var item in Model.Movies!)
        {
            <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>
                    @Html.DisplayFor(modelItem => item.Rating)
                </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>

Update the /Views/Movies/Create.cshtml with a Rating field.

Visual Studio / Visual Studio for Mac

You can copy/paste the previous "form group" and let intelliSense help you update the fields. IntelliSense works with Tag Helpers.

Visual Studio Code

Update the remaining templates.

Update the SeedData class so that it provides a value for the new column. A sample change is shown below, but you'll want to make this change for each new Movie.

new Movie
{
    Title = "When Harry Met Sally",
    ReleaseDate = DateTime.Parse("1989-1-11"),
    Genre = "Romantic Comedy",
    Rating = "R",
    Price = 7.99M
},

The app won't work until the DB is updated to include the new field. If it's run now, the following SqlException is thrown:

SqlException: Invalid column name 'Rating'.

This error occurs because the updated Movie model class is different than the schema of the Movie table of the existing database. (There's no Rating column in the database table.)

There are a few approaches to resolving the error:

1. Have the Entity Framework automatically drop and re-create the database based on the new model class schema. This approach is very convenient early in the development cycle when you're doing active development on a test database; it allows you to quickly evolve the model and database schema together. The downside, though, is that you lose existing data in the database — so you don't want to use this approach on a production database! Using an initializer to automatically seed a database with test data is often a productive way to develop an application. This is a good approach for early development and when using SQLite.

2. Explicitly modify the schema of the existing database so that it matches the model classes. The advantage of this approach is that you keep your data. You can make this change either manually or by creating a database change script.

3. Use Code First Migrations to update the database schema.

For this tutorial, Code First Migrations is used.

Visual Studio

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

In the PMC, enter the following commands:

Add-Migration Rating
Update-Database

The Add-Migration command tells the migration framework to examine the current Movie model with the current Movie DB schema and create the necessary code to migrate the DB to the new model.

The name "Rating" is arbitrary and is used to name the migration file. It's helpful to use a meaningful name for the migration file.

If all the records in the DB are deleted, the initialize method will seed the DB and include the Rating field.

Visual Studio Code / Visual Studio for Mac

Note

For this tutorial you use the Entity Framework Core migrations feature where possible. Migrations updates the database schema to match changes in the data model. However, migrations can only do the kinds of changes that the EF Core provider supports, and the SQLite provider's capabilities are limited. For example, adding a column is supported, but removing or changing a column is not supported. If a migration is created to remove or change a column, the ef migrations add command succeeds but the ef database update command fails. Due to these limitations, this tutorial doesn't use migrations for SQLite schema changes. Instead, when the schema changes, you drop and re-create the database.

The workaround for the SQLite limitations is to manually write migrations code to perform a table rebuild when something in the table changes. A table rebuild involves:

• Creating a new table.
• Copying data from the old table to the new table.
• Dropping the old table.
• Renaming the new table.

For more information, see the following resources:

• SQLite EF Core Database Provider Limitations
• Customize migration code
• Data seeding
• SQLite ALTER TABLE statement

Delete the Migrations folder and the database file, and then run the following .NET CLI commands:

dotnet ef migrations add InitialCreate

dotnet ef database update

For more information, see Resetting all migrations.

Run the app and verify you can create, edit, and display movies with a Rating field.

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