Entity Framework: Nakov @ BFU Hackhaton 2015

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

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

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

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

Entity Framework (EF)
The ORM Framework for .NET

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

Database First with Entity
Framework and Visual Studio
Live Demo

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
 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
 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
 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
 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
 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
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

CRUD Operations with EF
Live Demo

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

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

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

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
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

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

"Code First" Approach in EF
Live Demo

Using Code First Migrations in EF

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

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;
}
}

Using Code First Migrations in EF
Live Demo

?
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ORM Technologies and Entity Framework (EF)

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Entity Framework: Nakov @ BFU Hackhaton 2015

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