1. ORM Technologies and Entity Framework (EF)
Entity Framework, DbContext, CRUD Operations, Code First, Migrations
Svetlin Nakov
Inspiration Manager
Software University
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2. Table of Contents ORM Technologies – Basic Concepts
Entity Framework – Overview
Database First with EF
Reading Data and CRUD operations with EF
Code First with EF
Domain Classes and DbContext
Migrations in EF
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3. Introduction to ORM Technologies
What is Object-Relational Mapping (ORM)?

4. ORM Technologies
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

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

6. Object-Relation Persistence Framework for .NET
Entity Framework (EF)
Object-Relation Persistence Framework for .NET

7. Overview of EF
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

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

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

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

11. 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
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12. Reading Data with LINQ Query
We can also use extension methods for constructing the query
Find element by id
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
using (var context = new SoftUniEntities()) {
var project = context.Projects.Find(2);
Console.WriteLine(project.Name); }

13. Creating New Data
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
var project = new Project() {
Name = "Judge System",
StartDate = new DateTime(2015, 4, 15) };
context.Projects.Add(order);
context.SaveChanges();
Create a new project object
Mark the object for inserting
This will execute an SQL INSERT

14. Cascading Inserts 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
Employee employee = new Employee();
employee.FirstName = "Petya";
employee.LastName = "Grozdarska";
employee.Projects.Add(new Project { Name = "SoftUni Conf" } );
softUniEntities.Employees.Add(employee);
softUniEntities.SaveChanges();

15. Updating Existing Data
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
Employees employee =
softUniEntities.Employees.First();
employees.FirstName = "Alex";
context.SaveChanges();
This will execute an SQL SELECT to load the first order
This will execute an SQL UPDATE

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

17. Parameter placeholder
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
Return type
Parameter value

18. CRUD Operations with EF
Live Demo

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

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

21. DbContext & ModelBuilder
Code First in EF
Domain classes
DbContext & ModelBuilder DB
Custom Configuration
As needed
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22. 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. Domain Classes (Models) (2)
Another example of domain class (model)
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

24. Defining the DbContext Class
using System.Data.Entity;
using CodeFirst.Models;
public class ForumContext : 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

25. CRUD Operations with EF Code First
var db = new ForumContext();
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();
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26. "Code First" Approach in EF
Live Demo

27. Using Code First Migrations in EF

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

29. Using Code First Migrations in EF
Live Demo

30. ORM Technologies and Entity Framework (EF)
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31. Free Trainings @ Software University
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