The approach that you adopt to modify entities via the DbSet depends on whether the context is currently tracking the entity being modified or not.
In the following example, the entity is obtained by the context, so the context begins tracking it immediately. When you alter property values on a tracked entity, the context changes the EntityState for the entity to Modified and the ChangeTracker records the old property values and the new property values. When SaveChanges is called, an UPDATE statement is generated and executed by the database.
var author = context.Authors.Find(1);
author.FirstName = "Bill";
context.SaveChanges();
Since the ChangeTracker tracks which properties have been modified, the context will issue a SQL statement that updates only those properties that were changed:
exec sp_executesql N'SET NOCOUNT ON;
UPDATE [Authors] SET [FirstName] = @p0
WHERE [AuthorId] = @p1;
SELECT @@ROWCOUNT;
',N'@p1 int,@p0 nvarchar(4000)',@p1=1,@p0=N'Bill'
Disconnected Scenario
In a disconnected scenario such as an ASP.NET application, changes to an existing entity's property values can take place in a controller or service method, well away from the context. In these cases, the context needs to be informed that the entity is in a modified state. This can be achieved by using the DbSet<T>.Update method (which is new in EF Core).
DbSet Update
The DbSet<T> class provides Update and UpdateRange methods for working with an individual or multiple entities.
public void Save(Author author)
{
context.Authors.Update(author);
context.SaveChanges();
}
This method results in the entity being tracked by the context as Modified. The context doesn't have any way of identifying which property values have been changed and will generate SQL to update all properties. Any related entities (such as a collection of books in this example) will also be tracked in the Modified state, resulting in UPDATE statements being generated for each of them. If the related entity doesn't have a key value assigned, it will be marked as Added, and an INSERT statement will be generated instead.
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