DevTut

# Tracking vs. No-Tracking

# No-tracking queries

  • No tracking queries are useful when the results are used in a `read-only` scenario
  • They are `quicker to execute` because there is no need to setup change tracking information

    • Example :

    using (var context = new BookContext())
{
    var books = context.Books.AsNoTracking().ToList();
}

    With EF Core 1.0 you are also able to change the default tracking behavior at the context instance level.

    Example :

    using (var context = new BookContext())
{
    context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;

    var books = context.Books.ToList();
}

    # Tracking queries

    • By default, queries that return entity types are tracking
    • This means you can make changes to those entity instances and have those changes persisted by SaveChanges()

    Example :

  • The change to the `book` rating will be detected and persisted to the database during `SaveChanges()`.
    • 
 using (var context = new BookContext())
  {
    var book = context.Books.FirstOrDefault(b => b.BookId == 1);
    book.Rating = 5;
    context.SaveChanges();
  }

    # Tracking and projections

  • Even if the result type of the query isn’t an entity type, if the result `contains entity` types they will still be `tracked by default`

    • Example :

  • In the following query, which returns an `anonymous type`, the instances of `Book` in the result set `will be tracked`

     using (var context = new BookContext())
 {
    var book = context.Books.Select(b => new { Book = b, Authors = b.Authors.Count() });
 }

  • If the result set `does not` contain any `entity` types, then `no tracking` is performed

    • Example :

  • In the following query, which returns an `anonymous type` with some of the values from the entity (but `no instances` of the actual `entity` type), there is **no tracking** performed.

    using (var context = new BookContext())
{
   var book = context.Books.Select(b => new { Id = b.BookId, PublishedDate = b.Date });
}

    • # Remarks

    Tracking behavior controls whether or not Entity Framework will keep information about an entity instance in its change tracker. If an entity is tracked, any changes detected in the entity will be persisted to the database during SaveChanges().

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