Modeling Workflows

 Modeling Workflows

• Entity Framework supports three types of modeling workflow:

  • Database first

    • Create models as database tables

    • Use Management Studio or native SQL queries

  • Model first

    • Create models using visual EF designer in VS

  • Code first

    • Write models and combine them in DbContext

 Database First

• Create models as database tables and then generate code (models) from them

 Model First

• Create models as database diagrams and then generate code (models) and tables from them

 Code First

• Create models as code classes and then generate diagram and tables from them

 Why use Code First?

• Write code without having to define mappings in XML or create database models

• Define objects in POCO

  • Reuse these models and their attributes

• No base classes required

• Enables database persistence with no configuration

  • Can use automatic migrations

• Can use Data Annotations (Key, Required, etc.)

Code First

 Data Models

• Bunch of normal C# classes (POCO)

  • May contain navigation properties

public class PostAnswer
{
    public int PostAnswerId { get; set; } // Primary key
    public string Content { get; set; }

    public int PostId { get; set; } // Foreign key
    public virtual Post Post { get; set; } // Navigation property
}

 Data Models

• Another example of data class (model)

 DbContext

• A class that inherits from DbContext

  • Manages model classes using DbSet type

  • Implements identity tracking, change tracking, and API for CRUD operations

  • Provides LINQ-based data access

• Recommended to be in a separate class library

  • Don't forget to reference the Entity Framework library (using NuGet package manager)

• If you have a lot of models it is recommended to use more than one DbContext

 DbSet

• Collection of single entity type

• Set operations: Add, Attach, Remove, Find

• Use with DbContext to query database 

public DbSet<Post> Posts { get; set; }

public DbSet<PostAnswer> PostAnswers { get; set; }

 DbContext Example

using System.Data.Entity;
using CodeFirst.Models;

public class ForumContext : DbContext
{
    public IDbSet<Post> Posts { get; set; }

    public IDbSet<PostAnswer> PostAnswers { get; set; }

    public IDbSet<Category> Categories { get; set; }

    public IDbSet<Tag> Tags { get; set; }
}

 Data Interaction Example

var db = new ForumContext();

var category = new Category { Parent = null, Name = "Database course" };
db.Categories.Add(category);

var post = new Post();
post.Title = "Срока на домашните";
post.Content = "Моля удължете срока на домашните";
post.Type = PostType.Normal;
post.Category = category;
post.Tags.Add(new Tag { Text = "домашни" });
post.Tags.Add(new Tag { Text = "срок" });

db.Posts.Add(post);
db.SaveChanges();

 How to connect to SQL Server?

• First, create context constructor that calls base constructor with appropriate connection name

public class ForumContext : DbContext
{
    public ForumContext()
        : base("ForumDb")
    { }
    // ...
}

• Then add the connection string in app.config

<connectionStrings>
  <add name="ForumDb" connectionString="Data Source=.;Initial 
    Catalog=ForumDb;Integrated Security=True"
    providerName="System.Data.SqlClient" ></add>
</connectionStrings>

Code First Migrations

 Migrations

 Migrations

 Migration Strategies

 How to use?

Database.SetInitializer(
  new MigrateDatabaseToLatestVersion<ForumContext, Configuration>());

public Configuration()
{
    this.AutomaticMigrationsEnabled = true;
    this.AutomaticMigrationDataLossAllowed = true;
}

 Seed database

protected override void Seed(ForumContext context)
{
    /* This method will be called after migrating to
       the latest version. You can use the
       DbSet.AddOrUpdate() helper extension method 
       to avoid creating duplicate seed data. E.g. */

    context.Tags.AddOrUpdate(new Tag { Text = "срок" });
    context.Tags.AddOrUpdate(new Tag { Text = "форум" });
}

Configure Mappings

 Migrations

 Mappings

 Fluent API

protected override void OnModelCreating(
  DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Student>().HasKey(x => x.Id);
    modelBuilder.Entity<student>().Property(x => x.Name)
      .IsUnicode(true);
    modelBuilder.Entity<Student>().Property(x => x.Text)
      .HasMaxLength(255);

    base.OnModelCreating(modelBuilder);
}

 Fluent API

 Fluent API