The document provides an overview of Object-Relational Mapping (ORM) technologies and specifically focuses on the Entity Framework (EF) as the standard ORM framework for .NET. It covers fundamental concepts, CRUD operations, and migration strategies using EF, detailing both database-first and code-first approaches for data management and manipulation. Additionally, it includes practical examples and instructions for implementing EF in software development.

1. ORM Technologies and
Entity Framework (EF)
Entity Framework, DbContext, CRUD
Operations, Code First, Migrations
Svetlin Nakov
Inspiration Manager
Software University
http://softuni.bg

2. Table of Contents
1. ORM Technologies – Basic Concepts
2. Entity Framework – Overview
3. Database First with EF
4. Reading Data and CRUD operations with EF
5. Code First with EF
 Domain Classes and DbContext
6. Migrations in EF
2

3. Introduction to ORM Technologies
What is Object-Relational Mapping (ORM)?

4. 4
 Object-Relational Mapping (ORM) is a programming technique
for automatic mapping data and database schema
 Map relational DB tables to classes and objects
 ORM creates a "virtual object database"
 Used from the programming language (C#, Java, PHP, …)
 ORM frameworks automate the ORM process
 A.k.a. Object-Relational Persistence Frameworks
ORM Technologies

5. 5
ORM Mapping – Example
ORM
Framework
Relational
database
schema
ORM
Entities
(C# classes)

6. Entity Framework (EF)
The ORM Framework for .NET

7. 7
 Entity Framework (EF) is the standard ORM framework for .NET
 Maps relational database to C# object model
 Powerful data manipulation API over the mapped schema
 CRUD operations and complex querying with LINQ
 Database first approach: from database to C# classes
 Code first approach: from classes to DB schema
 Visual Studio generates EF data models
 Data mappings consist of C# classes, attributes and XML
Overview of EF

8. 8
EF: Basic Workflow
2. Write & execute
query over
IQueryable
3. EF generates &
executes an SQL
query in the DB
1. Define the data
model (use a DB
visual designer
or code first)

9. 9
EF: Basic Workflow (2)
5. Modify data
with C# code
and call "Save
Changes"
6. Entity Framework
generates &
executes SQL
command to
modify the DB
4. EF transforms
the query
results into
.NET objects

10. 10
 Install Entity Framework through the NuGet package manager
Installing Entity Framework

11. Database First with Entity
Framework and Visual Studio
Live Demo

12. Entity Framework – Components
 The DbContext class
 DbContext holds the DB connection
 Holds and DbSet<T> for the entity classes
 Provides LINQ-based data access ( through IQueryable)
 Provides API for CRUD operations
 Entity classes
 Hold entities (objects with their attributes and relations)
 Each database table is typically mapped to a single C# entity class
12

13. 13
 We can also use extension methods for constructing the query
 Find element by id
Reading Data with LINQ Query
using (var context = new SoftUniEntities())
{
var project = context.Projects.Find(2);
Console.WriteLine(project.Name);
}
using (var context = new SoftUniEntities())
{
var employees = context.Employees
.Select(c => c.FirstName)
.Where(c => c.JobTitle == "Design Engineering")
.ToList();
}
This is called projection
ToList() method
executes the SQL query
This is called
selection

14. 14
 To create a new database row use the method Add(…) of the
corresponding collection:
 SaveChanges() method executes the SQL insert / update /
delete commands in the database
Creating New Data
var project = new Project()
{
Name = "Judge System",
StartDate = new DateTime(2015, 4, 15)
};
context.Projects.Add(order);
context.SaveChanges(); This will execute an SQL INSERT
Create a new
project object
Mark the object for inserting

15. 15
 We can also add cascading entities to the database:
 This way we don't have to add Project individually
 They will be added when the Employee entity (employee) is
inserted to the database
Cascading Inserts
Employee employee = new Employee();
employee.FirstName = "Petya";
employee.LastName = "Grozdarska";
employee.Projects.Add(new Project { Name = "SoftUni Conf" } );
softUniEntities.Employees.Add(employee);
softUniEntities.SaveChanges();

16. 16
 DbContext allows modifying entity properties and persisting
them in the database
 Just load an entity, modify it and call SaveChanges()
 The DbContext automatically tracks all changes made on its
entity objects
Updating Existing Data
Employees employee =
softUniEntities.Employees.First();
employees.FirstName = "Alex";
context.SaveChanges(); This will execute
an SQL UPDATE
This will execute an
SQL SELECT to load
the first order

17. 17
 Delete is done by Remove() on the specified entity collection
 SaveChanges() method performs the delete action in the
database
Deleting Existing Data
Employees employee =
softUniEntities.Employees.First();
softUniEntities.Employees.Remove(employee);
softUniEntities.SaveChanges();
Mark the entity for
deleting at the next save
This will execute the
SQL DELETE command

18. 18
Native SQL Queries
var context = new SoftUniEntities();
string nativeSQLQuery =
"SELECT FirstName + ' ' + LastName " +
"FROM dbo.Employees WHERE JobTitle = {0}";
var employees = context.Database.SqlQuery<string>(
nativeSQLQuery, "Marketing Specialist");
foreach (var emp in employees)
{
Console.WriteLine(emp);
}
Parameter
placeholder
Parameter
value
Return
type

19. CRUD Operations with EF
Live Demo

20. "Code First" Approach in EF
From Classes to DB Schema

21. 21
 Create database schema and generate C# code (models) from it
Database First in EF
DB
EDMX
Model
Domain
Classes

22. Code First in EF
Custom
Configuration
DbContext &
ModelBuilder
Domain
classes
As needed
DB
22

23. Domain Classes (Models)
 Bunch of normal C# classes (POCO)
 May hold navigation properties
public class PostAnswer
{
public int Id { get; set; }
public string Content { get; set; }
public int PostId { get; set; }
public virtual Post Post { get; set; }
}
Primary key
Foreign key
Navigation property
Virtual for lazy loading
23

24. 24
 Another example of domain class (model)
Domain Classes (Models) (2)
public class Post
{
public Post()
{
this.Answers = new HashSet<PostAnswer>();
}
public virtual ICollection<PostAnswer> Answers { get; set; }
public PostType Type { get; set; }
}
This prevents
NullReferenceException
Navigation
property
Enumeration

25. 25
Defining the DbContext Class
using System.Data.Entity;
using CodeFirst.Models;
public class ForumDbContext : DbContext
{
public DbSet<Category> Categories { get; set; }
public DbSet<Post> Posts { get; set; }
public DbSet<PostAnswer> PostAnswers { get; set; }
public DbSet<Tag> Tags { get; set; }
}
Put all entity
classes as DbSets

26. CRUD Operations with EF Code First
var db = new ForumDbContext();
var category = new Category { Name = "Database course" };
db.Categories.Add(category);
var post = new Post();
post.Title = "Homework Deadline";
post.Content = "Please extend the homework deadline";
post.Type = PostType.Normal;
post.Category = category;
post.Tags.Add(new Tag { Text = "homework" });
post.Tags.Add(new Tag { Text = "deadline" });
db.Posts.Add(post);
db.SaveChanges();
26

27. "Code First" Approach in EF
Live Demo

28. Using Code First Migrations in EF

29. 29
 Enable Code First Migrations
 Open Package Manager Console
 Run Enable-Migrations command
 -EnableAutomaticMigrations for auto migrations
Code First Migrations in Entity Framework

30. 30
Configuring the Migration Strategy
// Enable automatic DB migrations for ForumDbContext
Database.SetInitializer(
new MigrateDatabaseToLatestVersion<
ForumDbContext, DbMigrationConfig>());
class DbMigrationConfig :
DbMigrationsConfiguration<ForumDbContext>
{
public Configuration()
{
AutomaticMigrationsEnabled = true;
AutomaticMigrationDataLossAllowed = true;
}
}

31. Using Code First Migrations in EF
Live Demo

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ORM Technologies and Entity Framework (EF)

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