Part 6, Razor Pages with EF Core in ASP.NET Core - Read Related Data

By Tom Dykstra, Jon P Smith, and Rick Anderson

The Contoso University web app demonstrates how to create Razor Pages web apps using EF Core and Visual Studio. For information about the tutorial series, see the first tutorial.

If you run into problems you can't solve, download the completed app and compare that code to what you created by following the tutorial.

This tutorial shows how to read and display related data. Related data is data that EF Core loads into navigation properties.

The following illustrations show the completed pages for this tutorial:

Eager, explicit, and lazy loading

There are several ways that EF Core can load related data into the navigation properties of an entity:

• Eager loading. Eager loading is when a query for one type of entity also loads related entities. When an entity is read, its related data is retrieved. This typically results in a single join query that retrieves all of the data that's needed. EF Core will issue multiple queries for some types of eager loading. Issuing multiple queries can be more efficient than a large single query. Eager loading is specified with the Include and ThenInclude methods.

  

  Eager loading sends multiple queries when a collection navigation is included:

  • One query for the main query
  • One query for each collection "edge" in the load tree.

• Separate queries with Load: The data can be retrieved in separate queries, and EF Core "fixes up" the navigation properties. "Fixes up" means that EF Core automatically populates the navigation properties. Separate queries with Load is more like explicit loading than eager loading.

  

  Note: EF Core automatically fixes up navigation properties to any other entities that were previously loaded into the context instance. Even if the data for a navigation property is not explicitly included, the property may still be populated if some or all of the related entities were previously loaded.

• Explicit loading. When the entity is first read, related data isn't retrieved. Code must be written to retrieve the related data when it's needed. Explicit loading with separate queries results in multiple queries sent to the database. With explicit loading, the code specifies the navigation properties to be loaded. Use the Load method to do explicit loading. For example:

  

• Lazy loading. When the entity is first read, related data isn't retrieved. The first time a navigation property is accessed, the data required for that navigation property is automatically retrieved. A query is sent to the database each time a navigation property is accessed for the first time. Lazy loading can hurt performance, for example when developers use N+1 queries. N+1 queries load a parent and enumerate through children.

Create Course pages

The Course entity includes a navigation property that contains the related Department entity.

To display the name of the assigned department for a course:

• Load the related Department entity into the Course.Department navigation property.
• Get the name from the Department entity's Name property.

Scaffold Course pages

Visual Studio

• Follow the instructions in Scaffold Student pages with the following exceptions:

  • Create a Pages/Courses folder.
  • Use Course for the model class.
  • Use the existing context class instead of creating a new one.

Visual Studio Code

• Create a Pages/Courses folder.

• Run the following command to scaffold the Course pages.

  On Windows:

  dotnet aspnet-codegenerator razorpage -m Course -dc SchoolContext -udl -outDir Pages\Courses --referenceScriptLibraries
  

  On Linux or macOS:

  dotnet aspnet-codegenerator razorpage -m Course -dc SchoolContext -udl -outDir Pages/Courses --referenceScriptLibraries
  

• Open Pages/Courses/Index.cshtml.cs and examine the OnGetAsync method. The scaffolding engine specified eager loading for the Department navigation property. The Include method specifies eager loading.

• Run the app and select the Courses link. The department column displays the DepartmentID, which isn't useful.

Display the department name

Update Pages/Courses/Index.cshtml.cs with the following code:

using ContosoUniversity.Models;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.EntityFrameworkCore;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace ContosoUniversity.Pages.Courses
{
    public class IndexModel : PageModel
    {
        private readonly ContosoUniversity.Data.SchoolContext _context;

        public IndexModel(ContosoUniversity.Data.SchoolContext context)
        {
            _context = context;
        }

        public IList<Course> Courses { get; set; }

        public async Task OnGetAsync()
        {
            Courses = await _context.Courses
                .Include(c => c.Department)
                .AsNoTracking()
                .ToListAsync();
        }
    }
}

The preceding code changes the Course property to Courses and adds AsNoTracking.

No-tracking queries are useful when the results are used in a read-only scenario. They're generally quicker to execute because there's no need to set up the change tracking information. If the entities retrieved from the database don't need to be updated, then a no-tracking query is likely to perform better than a tracking query.

In some cases a tracking query is more efficient than a no-tracking query. For more information, see Tracking vs. No-Tracking Queries. In the preceding code, AsNoTracking is called because the entities aren't updated in the current context.

Update Pages/Courses/Index.cshtml with the following code.

@page
@model ContosoUniversity.Pages.Courses.IndexModel

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

<h1>Courses</h1>

<p>
    <a asp-page="Create">Create New</a>
</p>
<table class="table">
    <thead>
        <tr>
            <th>
                @Html.DisplayNameFor(model => model.Courses[0].CourseID)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Courses[0].Title)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Courses[0].Credits)
            </th>
            <th>
                @Html.DisplayNameFor(model => model.Courses[0].Department)
            </th>
            <th></th>
        </tr>
    </thead>
    <tbody>
@foreach (var item in Model.Courses)
{
        <tr>
            <td>
                @Html.DisplayFor(modelItem => item.CourseID)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Title)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Credits)
            </td>
            <td>
                @Html.DisplayFor(modelItem => item.Department.Name)
            </td>
            <td>
                <a asp-page="./Edit" asp-route-id="@item.CourseID">Edit</a> |
                <a asp-page="./Details" asp-route-id="@item.CourseID">Details</a> |
                <a asp-page="./Delete" asp-route-id="@item.CourseID">Delete</a>
            </td>
        </tr>
}
    </tbody>
</table>

The following changes have been made to the scaffolded code:

• Changed the Course property name to Courses.

• Added a Number column that shows the CourseID property value. By default, primary keys aren't scaffolded because normally they're meaningless to end users. However, in this case the primary key is meaningful.

• Changed the Department column to display the department name. The code displays the Name property of the Department entity that's loaded into the Department navigation property:

  @Html.DisplayFor(modelItem => item.Department.Name)
  

Run the app and select the Courses tab to see the list with department names.

Loading related data with Select

The OnGetAsync method loads related data with the Include method. The Select method is an alternative that loads only the related data needed. For single items, like the Department.Name it uses a SQL INNER JOIN. For collections, it uses another database access, but so does the Include operator on collections.

The following code loads related data with the Select method:

public IList<CourseViewModel> CourseVM { get; set; }

public async Task OnGetAsync()
{
    CourseVM = await _context.Courses
    .Select(p => new CourseViewModel
    {
        CourseID = p.CourseID,
        Title = p.Title,
        Credits = p.Credits,
        DepartmentName = p.Department.Name
    }).ToListAsync();
}

The preceding code doesn't return any entity types, therefore no tracking is done. For more information about the EF tracking, see Tracking vs. No-Tracking Queries.

The CourseViewModel:

public class CourseViewModel
{
    public int CourseID { get; set; }
    public string Title { get; set; }
    public int Credits { get; set; }
    public string DepartmentName { get; set; }
}

See IndexSelectModel for the complete Razor Pages.

Create Instructor pages

This section scaffolds Instructor pages and adds related Courses and Enrollments to the Instructors Index page.

This page reads and displays related data in the following ways:

• The list of instructors displays related data from the OfficeAssignment entity (Office in the preceding image). The Instructor and OfficeAssignment entities are in a one-to-zero-or-one relationship. Eager loading is used for the OfficeAssignment entities. Eager loading is typically more efficient when the related data needs to be displayed. In this case, office assignments for the instructors are displayed.
• When the user selects an instructor, related Course entities are displayed. The Instructor and Course entities are in a many-to-many relationship. Eager loading is used for the Course entities and their related Department entities. In this case, separate queries might be more efficient because only courses for the selected instructor are needed. This example shows how to use eager loading for navigation properties in entities that are in navigation properties.
• When the user selects a course, related data from the Enrollments entity is displayed. In the preceding image, student name and grade are displayed. The Course and Enrollment entities are in a one-to-many relationship.

Create a view model

The instructors page shows data from three different tables. A view model is needed that includes three properties representing the three tables.

Create Models/SchoolViewModels/InstructorIndexData.cs with the following code:

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

namespace ContosoUniversity.Models.SchoolViewModels
{
    public class InstructorIndexData
    {
        public IEnumerable<Instructor> Instructors { get; set; }
        public IEnumerable<Course> Courses { get; set; }
        public IEnumerable<Enrollment> Enrollments { get; set; }
    }
}

Scaffold Instructor pages

Visual Studio

• Follow the instructions in Scaffold the student pages with the following exceptions:

  • Create a Pages/Instructors folder.
  • Use Instructor for the model class.
  • Use the existing context class instead of creating a new one.

Visual Studio Code

• Create a Pages/Instructors folder.

• Run the following command to scaffold the Instructor pages.

  On Windows:

  dotnet aspnet-codegenerator razorpage -m Instructor -dc SchoolContext -udl -outDir Pages\Instructors --referenceScriptLibraries
  

  On Linux or macOS:

  dotnet aspnet-codegenerator razorpage -m Instructor -dc SchoolContext -udl -outDir Pages/Instructors --referenceScriptLibraries
  

Run the app and navigate to the Instructors page.

Update Pages/Instructors/Index.cshtml.cs with the following code:

using ContosoUniversity.Models;
using ContosoUniversity.Models.SchoolViewModels;  // Add VM
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.EntityFrameworkCore;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace ContosoUniversity.Pages.Instructors
{
    public class IndexModel : PageModel
    {
        private readonly ContosoUniversity.Data.SchoolContext _context;

        public IndexModel(ContosoUniversity.Data.SchoolContext context)
        {
            _context = context;
        }

        public InstructorIndexData InstructorData { get; set; }
        public int InstructorID { get; set; }
        public int CourseID { get; set; }

        public async Task OnGetAsync(int? id, int? courseID)
        {
            InstructorData = new InstructorIndexData();
            InstructorData.Instructors = await _context.Instructors
                .Include(i => i.OfficeAssignment)                 
                .Include(i => i.Courses)
                    .ThenInclude(c => c.Department)
                .OrderBy(i => i.LastName)
                .ToListAsync();

            if (id != null)
            {
                InstructorID = id.Value;
                Instructor instructor = InstructorData.Instructors
                    .Where(i => i.ID == id.Value).Single();
                InstructorData.Courses = instructor.Courses;
            }

            if (courseID != null)
            {
                CourseID = courseID.Value;
                IEnumerable<Enrollment> Enrollments = await _context.Enrollments
                    .Where(x => x.CourseID == CourseID)                    
                    .Include(i=>i.Student)
                    .ToListAsync();                 
                InstructorData.Enrollments = Enrollments;
            }
        }
    }
}

The OnGetAsync method accepts optional route data for the ID of the selected instructor.

Examine the query in the Pages/Instructors/Index.cshtml.cs file:

InstructorData = new InstructorIndexData();
InstructorData.Instructors = await _context.Instructors
    .Include(i => i.OfficeAssignment)                 
    .Include(i => i.Courses)
        .ThenInclude(c => c.Department)
    .OrderBy(i => i.LastName)
    .ToListAsync();

The code specifies eager loading for the following navigation properties:

• Instructor.OfficeAssignment
• Instructor.Courses
  • Course.Department

The following code executes when an instructor is selected, that is, id != null.

if (id != null)
{
    InstructorID = id.Value;
    Instructor instructor = InstructorData.Instructors
        .Where(i => i.ID == id.Value).Single();
    InstructorData.Courses = instructor.Courses;
}

The selected instructor is retrieved from the list of instructors in the view model. The view model's Courses property is loaded with the Course entities from the selected instructor's Courses navigation property.

The Where method returns a collection. In this case, the filter select a single entity, so the Single method is called to convert the collection into a single Instructor entity. The Instructor entity provides access to the Course navigation property.

The Single method is used on a collection when the collection has only one item. The Single method throws an exception if the collection is empty or if there's more than one item. An alternative is SingleOrDefault, which returns a default value if the collection is empty. For this query, null in the default returned.

The following code populates the view model's Enrollments property when a course is selected:

if (courseID != null)
{
    CourseID = courseID.Value;
    IEnumerable<Enrollment> Enrollments = await _context.Enrollments
        .Where(x => x.CourseID == CourseID)                    
        .Include(i=>i.Student)
        .ToListAsync();                 
    InstructorData.Enrollments = Enrollments;
}

Update the instructors Index page

Update Pages/Instructors/Index.cshtml with the following code.

@page "{id:int?}"
@model ContosoUniversity.Pages.Instructors.IndexModel

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

<h2>Instructors</h2>

<p>
    <a asp-page="Create">Create New</a>
</p>
<table class="table">
    <thead>
        <tr>
            <th>Last Name</th>
            <th>First Name</th>
            <th>Hire Date</th>
            <th>Office</th>
            <th>Courses</th>
            <th></th>
        </tr>
    </thead>
    <tbody>
        @foreach (var item in Model.InstructorData.Instructors)
        {
            string selectedRow = "";
            if (item.ID == Model.InstructorID)
            {
                selectedRow = "table-success";
            }
            <tr class="@selectedRow">
                <td>
                    @Html.DisplayFor(modelItem => item.LastName)
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.FirstMidName)
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.HireDate)
                </td>
                <td>
                    @if (item.OfficeAssignment != null)
                    {
                        @item.OfficeAssignment.Location
                    }
                </td>
                <td>
                    @{
                        foreach (var course in item.Courses)
                        {
                            @course.CourseID @:  @course.Title <br />
                        }
                    }
                </td>
                <td>
                    <a asp-page="./Index" asp-route-id="@item.ID">Select</a> |
                    <a asp-page="./Edit" asp-route-id="@item.ID">Edit</a> |
                    <a asp-page="./Details" asp-route-id="@item.ID">Details</a> |
                    <a asp-page="./Delete" asp-route-id="@item.ID">Delete</a>
                </td>
            </tr>
        }
    </tbody>
</table>

@if (Model.InstructorData.Courses != null)
{
    <h3>Courses Taught by Selected Instructor</h3>
    <table class="table">
        <tr>
            <th></th>
            <th>Number</th>
            <th>Title</th>
            <th>Department</th>
        </tr>

        @foreach (var item in Model.InstructorData.Courses)
        {
            string selectedRow = "";
            if (item.CourseID == Model.CourseID)
            {
                selectedRow = "table-success";
            }
            <tr class="@selectedRow">
                <td>
                    <a asp-page="./Index" asp-route-courseID="@item.CourseID">Select</a>
                </td>
                <td>
                    @item.CourseID
                </td>
                <td>
                    @item.Title
                </td>
                <td>
                    @item.Department.Name
                </td>
            </tr>
        }

    </table>
}

@if (Model.InstructorData.Enrollments != null)
{
    <h3>
        Students Enrolled in Selected Course
    </h3>
    <table class="table">
        <tr>
            <th>Name</th>
            <th>Grade</th>
        </tr>
        @foreach (var item in Model.InstructorData.Enrollments)
        {
            <tr>
                <td>
                    @item.Student.FullName
                </td>
                <td>
                    @Html.DisplayFor(modelItem => item.Grade)
                </td>
            </tr>
        }
    </table>
}

The preceding code makes the following changes:

• Updates the page directive to @page "{id:int?}". "{id:int?}" is a route template. The route template changes integer query strings in the URL to route data. For example, clicking on the Select link for an instructor with only the @page directive produces a URL like the following:

  https://localhost:5001/Instructors?id=2

  When the page directive is @page "{id:int?}", the URL is: https://localhost:5001/Instructors/2

• Adds an Office column that displays item.OfficeAssignment.Location only if item.OfficeAssignment isn't null. Because this is a one-to-zero-or-one relationship, there might not be a related OfficeAssignment entity.

  @if (item.OfficeAssignment != null)
  {
      @item.OfficeAssignment.Location
  }
  

• Adds a Courses column that displays courses taught by each instructor. See Explicit line transition for more about this razor syntax.

• Adds code that dynamically adds class="table-success" to the tr element of the selected instructor and course. This sets a background color for the selected row using a Bootstrap class.

  string selectedRow = "";
  if (item.CourseID == Model.CourseID)
  {
      selectedRow = "table-success";
  }
  <tr class="@selectedRow">
  

• Adds a new hyperlink labeled Select. This link sends the selected instructor's ID to the Index method and sets a background color.

  <a asp-action="Index" asp-route-id="@item.ID">Select</a> |
  

• Adds a table of courses for the selected Instructor.

• Adds a table of student enrollments for the selected course.

Run the app and select the Instructors tab. The page displays the Location (office) from the related OfficeAssignment entity. If OfficeAssignment is null, an empty table cell is displayed.

Click on the Select link for an instructor. The row style changes and courses assigned to that instructor are displayed.

Select a course to see the list of enrolled students and their grades.

Next steps

The next tutorial shows how to update related data.

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