Data modification

Each context instance has a ChangeTracker that is responsible for keeping track of changes that need to be written to the database. As you make changes to instances of your entity classes, these changes are recorded in the ChangeTracker and then written to the database when you call SaveChanges. The database provider is responsible for translating the changes into database-specific operations (for example, INSERT, UPDATE, and DELETE commands for a relational database).

Save data

using (var context = new BloggingContext())
{
    var blog = new Blog { Url = "http://sample.com" };
    context.Blogs.Add(blog);
    context.SaveChanges();
}

Related data

If you create several new related entities, adding one of them to the context will cause the others to be added too.

using (var context = new BloggingContext())
{
    var blog = new Blog
    {
        Url = "http://blogs.msdn.com/dotnet",
        Posts = new List<Post>
        {
            new Post { Title = "Intro to C#" },
            new Post { Title = "Intro to VB.NET" },
            new Post { Title = "Intro to F#" }
        }
    };

    context.Blogs.Add(blog);
    context.SaveChanges();
}

Cascade delete

Cascade delete is commonly used in database terminology to describe a characteristic that allows the deletion of a row to automatically trigger the deletion of related rows. A closely related concept also covered by EF Core delete behaviors is the automatic deletion of a child entity when it's relationship to a parent has been severed--this is commonly known as "deleting orphans".

Optional relationships

For optional relationships (nullable foreign key) it is possible to save a null foreign key value, which results in the following effects:

Behavior Name	Effect on dependent/child in memory	Effect on dependent/child in database
Cascade	Entities are deleted	Entities are deleted
ClientSetNull (Default)	Foreign key properties are set to null	None
SetNull	Foreign key properties are set to null	Foreign key properties are set to null
Restrict	None	None

Required relationships

For required relationships (non-nullable foreign key) it is not possible to save a null foreign key value, which results in the following effects:

Behavior Name	Effect on dependent/child in memory	Effect on dependent/child in database
Cascade (Default)	Entities are deleted	Entities are deleted
ClientSetNull	SaveChanges throws	None
SetNull	SaveChanges throws	SaveChanges throws
Restrict	None	None

Disconnected entities

A DbContext instance will automatically track entities returned from the database.

However, sometimes entities are queried using one context instance and then saved using a different instance. This often happens in "disconnected" scenarios.

Attach

The DbSet.Attach() method attaches an entire entity graph to the new context with the Unchanged entity state.

//disconnected entity graph
Student disconnectedStudent = new Student() { StudentName = "New Student" };
disconnectedStudent.StudentAddress = new StudentAddress() { Address1 = "Address", City = "City1" };

using (var context = new SchoolDBEntities())
{
    context.Students.Attach(disconnectedStudent);
                
    // get DbEntityEntry instance to check the EntityState of specified entity
    var studentEntry = context.Entry(disconnectedStudent);
    var addressEntry = context.Entry(disconnectedStudent.StudentAddress);

    Console.WriteLine("Student: {0}",studentEntry.State);
    Console.WriteLine("StudentAddress: {0}",addressEntry.State);
}