EF Core Save Data

To only update one field, we can simply change the update method to the following:

Person person = new Person {Id=4, Lastname="Miller"};
dbContext.Attach(person);
dbContext.Entry(person).Property(p => p.Lastname).IsModified = true;
dbContext.SaveChanges();

The above function first constructs the object with the specified Id and updated Lastname, and then appends the object; it then explicitly marks the Lastname property as modified.  

The generated UPDATE statement now looks like this

info: Microsoft.EntityFrameworkCore.Database.Command[20101]
      Executed DbCommand (8ms) [Parameters=[@p1='?' (DbType = Int32), @p0='?' (Size = 4000)], CommandType='Text', CommandTimeout='30']
      UPDATE `Persons` SET `Lastname` = @p0
      WHERE `Id` = @p1;
      SELECT ROW_COUNT();

As shown in the EFCore log, only the Lastname field is updated.  

This approach can slightly improve performance because the SQL statement is smaller and the query execution on the database server can be faster.

See also the discussion at Stack Overflow How to update not every fields of an object using Entity Framework and EntityState.Modified

Make EF Core update only the updated properties by turning the disconnected scenario to connected.

public void SaveBook(Book book)
{
    // Here, 'book' is the book with the changed Title.
    using(var context = new TestContext())
    {
        var dbBook = context.Books.Find(book.ID);

        // Copy book's property values to dbBook.
        context.Entry(dbBook).CurrentValues.SetValues(book);

        context.SaveChanges();
    }
}

There may be good reasons to prefer the latter method above the former.

This code sample show how to insert an entity Element without updating related entities.

_dbContext.Entry<Element>(element).State = EntityState.Added;

// Set the state of the Category navigation property of 'element' to Unchanged
_dbContext.Entry<Category>(element.Category).State = EntityState.Unchanged;

// Detach all roles associated with the 'element' from the DbContext
element.Roles.ToList().ForEach(r => _dbContext.Entry<Role>(r).State = EntityState.Detached);

// Mark the Roles collection of 'element' as not modified to prevent updating roles
_dbContext.Entry<Element>(element).Collection(r => r.Roles).IsModified = false;

await _dbContext.SaveChangesAsync();

• The element's associated Category object to Unchanged in the _dbContext. This indicates that the Category object is not modified and should not be updated in the database.
• For each Role, it sets the state to Detached. Detaching an entity means it is no longer tracked by the EF Core context for changes.
• The IsModified property of the Roles collection in the element is explicitly set to false. This indicates to EF Core that the Roles collection has not been modified and should not be updated in the database.