EF Core ChangeTracker

When you call the SaveChanges method on the DbContext, the context needs to be able to generate the appropriate commands for each object that it is currently tracking. So it needs to know about the "state" of each object - whether it is new, or it is an existing object that has been modified in some way, or whether it is scheduled for deletion. The Change Tracker is the mechanism responsible for this process.

The Change Tracker records the current state of an entity using one of four values:

• Added
• Unchanged
• Modified
• Deleted

Entities in the Added state will be inserted as new records into the database. Entities in the Modified state will have their values updated in the database to the current property values. Entities in the Deleted state will be removed from the database. No action will be taken in respect of entities marked as Unchanged.

Change tracking begins as soon as an entity is loaded. An entity is loaded as a result of being returned from a query or by being introduced to the context through one of the following methods of the DbContext

• Add
• Attach
• Update
• Remove

or by having its State property set on the entity entry returned by calling the context's Entry method.

DbContext.Add

Tells EF that the entity is new and should be inserted into the database, and so sets the state to Added. It also does this for all reachable entities in the graph that it isn't already tracking.

var db = new TestContext();
var book = new Book { 
    Title = "King Lear",
    Author = new Author { 
        FirstName = "William",
        LastName = "Shakespeare"
    }
}
db.Add(book); // Book Added, Author Added

var db = new TestContext();
var book = new Book { Title = "Romeo and Juliet" };
book.Author = db.Authors.Single(a => a.AuthorId == 1);
db.Add(book); // Book Added, Author Unchanged

var db = new TestContext();
var book = new Book { Title = "Romeo and Juliet" }; // Book Added, 
book.Author = db.Authors.Single(a => a.AuthorId == 1); // Author Unchanged
book.Author.FirstName = "Bill"; // Author Modified
db.Add(book);

var db = new TestContext();
var book = new Book { Title = "Hamlet", AuthorId = 1 }; 
db.Add(book); // Book Added - only one entry being tracked

DbContext.Attach

Tells EF that the entity already exists in the database, and sets the state of the entity to Unchanged.

var db = new TestContext();
var book = new Book { BookId = 1 }; 
db.Attach(book); // Book Unchanged

var db = new TestContext();
var book = new Book { 
    BookId = 1, 
    Author = new Author {
        FirstName = "Charles", 
        LastName = "Dickens" 
    } 
};
db.Attach(book); // Book Modified, Author Added - no "store generated key"

DbContext.Update

Tells EF that the entity is modified. It will also set all reachable entities as modified if they are not already being tracked.

var db = new TestContext();
var book = new Book { BookId = 1 }; 
db.Update(book); // Book Modified

DbContext.Remove

Marks only the root entity as deleted. Cascade deletion will take effect if implemented.

var db = new TestContext();
var book = new Book { BookId = 1 };
db.Remove(book); // Book Deleted

var db = new TestContext();
var book = new Book {
    BookId = 1, 
    Author = db.Authors.SingleOrDefault(a => a.AuthorId == 1) 
};
db.Remove(book); // Book Deleted,  Author Unchanged

DbContext.Entry.State

Only sets the state for that entry. Others in the graph are unaffected.