ict.ken.be

Entity Framework 4.1 DbContext Data Access
Julie Lerman

1. DbContext

• .Add
• .Attach is not an update but tells EF to track the entity.
• .Remove
• .Entry.State=Modified Added Deleted

var newPerson = context.People.Add(new Person { Name = "Julie"});
Assert.IsTrue(context.Entry(newPerson).State==System.Data.EnitityState.Added);
 
//context.Aliases.Attach(existingAlias); //line below willl do implicit
context.Entry(existingAlias).State = System.Data.EntityState.Modified;
Assert.IsTrue(context.Entry(exisitingAlias).State == System.Data.EntityState.Modified);
 
var query = from p in context.Person where p.Id == 3 select p;
var person = query.FirstOrDefault();
//use instead ->
var person = context.People.Find(3); //uses context cach as a bonus
 
//objects managed by the cache
ObservableCollection<Person> = context.People.Local 

[TestMethod]
public void ObjectsGetDynamicProxies()
{
	var context = new TwitterContext();
	var alias = context.Aliases.FirstOrDefault();
	Assert.IsTrue(alias.GetType().FullName.StartsWith("System.Data.Entity.DynamicProxies"));
}
 
[TestMethod]
public void ObjectsAreInstantlyChangeTracked()
{
	var context = new TwitterContext();
	var alias = context.Aliases.FirstOrDefault();
	alias.Name = "abcde";
	//context.ChangeTracker.DetectChanges(); when not using dynamic proxy
	Assert.IsTrue(((context as IObjectContextAdapter).ObjectContext).ObjectStateManager.GetObjectStateEntries(EntityState.Modified).Count() == 1);
}
 
public class Alias
{
	public virtual int AuthorKey { get; set; } //will create lazy loading dynamic proxy
	public virtual ICollection<Tweet> Tweets { get; set; } //one marked as virtual will enable lazy loading
	public string Email { get; set; } //must be virual if you want change tracking
	public string AliasPlusName //no setter so will be skipped
	{
		get { return Name + "(" + UserName + ")"; }
	}
}

• DetectChanges called by:
• DbSet .Find .Local .Remove .Add .Attach
• DbContext .SaveChanges .GetValidationErrors .Entry
• DbChangeTracker.Entries

//if you do NOT want DetectChanges to get called
context.Configuration.AutoDetectChangesEnabled = false;

• var oContext = (myDbContext as ObjectContextAdapter).ObjectContext;
• Linq to Entities CompiledQuery: No DbContext, No Adapter.ObjectContext
• EF5 : http://blogs.msdn.com/b/stuartleeks/archive/2012/06/12/entity-framework-5-controlling-automatic-query-compilation.aspx

[TestMethod]
public void CanGetDatabaseValuesFromEntry()
{
	var context = new TwitterContext();
	var alias = context.Aliases.FirstOrDefault();
	DbPropertyValues dbValues = context.Entry(alias).GetDatabaseValues();
	Assert.IsNotNull(dbValues);			
}
 
//when savechanges and an optimistic concurrency error was thrown
var entry = context.Entry(alias);
entry.OriginalValues.SetValues(entry.GetDatabaseValues()); //client win
entry.Reload(); //or use for server win

• edmx file -> DbContext T4 Template -> Simple POCO Classes and DbContext
• ADO.Net DbContext Generation 

2. Code First Database Initialization

• Connectionstring Available (else use buildstring) -> Database Exists (else create database) -> Use Database
• Buildstring: SQL Server Express using Context Strong Name
• eg. namespace DAL class myContext -> DAL.myContext will be the database name

public class MyContext : DbContext
{
  public MyContext() : base("MyOwnDatabaseName") {}
 
  protected override void OnModelCreating(DbModelBuilder modelBuilder)
  {
	modelBuilder.Ignore<Privacy>();
	modelBuilder.Configurations.Add(new AliasConfiguration());	
}
}

1. Read classes & configurations
2. Build in-memory Model & mappings
3. Compare model to EdmMetadata table
4. Changes? Follow init strategy (default: throw exception)

//on application_start()
Database.SetInitializer(new DropCreateDatabaseIfModelChanges<MyContext>());

• CreateDataseIfNotExists
• DropCreateDatabaseIfModelChanges
• DropCreateDatabaseAlways
• YourCustomDBInitialize

• http://msdn.microsoft.com/en-us/data/JJ591621.aspx

public class MyInitializer : DropCreateDatabaseIfModelChanges<MyContext>
{
  protected override void Seed(TwitterContext context)
  {
 	new List<Alias> { ... }.ForEach(b => context.Aliases.Add(b));
	base.Seed(context); //calls SaveChanges, so you don't have to
  }
}
Database.SetInitializer(new MyInitializer());

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

• DbModelBuilder.Conventions.Remove(IncludeMetadataConvention);
• Database.SetInitialize<MyContext>(null);
• <appSettings><add key="DatabaseInitializerForType DAL.MyContext, DataAccessAssembly" value="Disabled" /></appSettings>

• IList<T> will be IList`1 and IList[]
• IDictionary<K,V> will be IDictionary`2 and IDictionary[,]

• Create, CreateIfNotExists, Delete, Exists
• GetHashCode
• ExecuteSqlCommand, SqlQuery
• CompatibleWithModel
• Initialize, SetInitializer(TContext)
• Connection, DefaultConnectionFactory

3. More on Entity Framework

• http://msdn.com/data/ef
• EF Team - http://blogs.msdn.com/adonet
• Rowan Miller - http://romiller.com
• Arthur Vickers - http://blog.oneunicorn.com
• http://learnentityframework.com