Entity Framework

gistfile1.txt

https://www.learnentityframeworkcore.com/

1. Adding 
2. Modifying

a) 
var author = context.Authors.First(a => a.AuthorId == 1);
author.FirstName = "Bill";
context.SaveChanges();

Generates sql that updates only FirstName prop of author entity

b)

public void Save(Author author)
{
    context.Entry(author).State = EntityState.Modified;
    context.SaveChanges();
}

Any related objects will not be tracked (props of Author, like List<Book> books)
Generate SQL that updates all properties (except primary key)


c) 

context.Update() 
context.SaveChanges()

Update all properties. Related objects will be updated too. For related objects without
Primary key will be generated INSERT statement 

d) Attach

When atatch, state will be set to Unchanged. (no sql code generated at all)
Reachable entities without key values => marked as Added
However, now the entityis being tracked by context, so can inform which properties were
modified (and proper sql will be generated):

context.Attach(author);
context.Entry(author).Property("FirstName").IsModified = true;

e) TrackGraph
Complex object.
For example:
it is assumed that the author entity has not been changed, 
but the books might have been edited

// author is not tracked by EF yet
context.ChangeTracker.TrackGraph(author, e => { 
    if((e.Entry.Entity as Author) != null) 
        e.Entry.State = EntityState.Unchanged //EF now tracks the author entity 
    else
        e.Entry.State = EntityState.Modified; //If author property is Books 
});

=> update all the books field by field

3. Deleting 

a) c
context.Remove(context.Authors.Single(a => a.AuthorId == 1));
context.SaveChanges();

Produces 2 SQL queries : select & delete

b) 
context.Remove(new Author { AuthorId = 1 };);
context.SaveChanges();

Creates stub and delete. Execute only 1 request.

c) context.Entry(author).State = EntityState.Deleted;

Related Data:
If the entity that you want to delete has related data,
the approach that you take will depend on how the relationship has been configured.
(if configured in sql or by fluent api)

4. Change Tracker

Added
Unchanged
Modified
Deleted

DbSets:

QueryingData

Find - shorthand (syntactic sugar) for the SingleOrDefault method
^ May return null.
Single, FirstOrDefault - execute's immediately

GroupBy - in EF 2.1 executes in-memory.

context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;
^ instead of .AsNotTracking() in each read-request

AddingData 


[ConcurrencyCheck]
При обновлении, добавит в where предикат, со значением поля на момент извлечения.
Т.е

class Model {
    [ConcurrencyCheck]
    public string BloggerName { get; set; }
}

where (([PrimaryTrackingKey] = @4) and ([BloggerName] = @5))
@4=1,@5=N'Julie'                          ^ !!!!!!!!!!!!

Если значение изменится, то DbUpdateConcurrencyException


[TimeStamp]
Тоже для concurrency checking. 
byte[], не nullable


[DatabaseGenerated(DatabaseGeneratedOption.Computed)]

InverseProperty
^ Если два объекта одного типа в классе, то EF не может сам разобраться
The InverseProperty is used when you have multiple relationships between classes.

EF кэширует результат операции Find.
var blog = context.Blogs.Find(3);

// Will return the same instance without hitting the database
var blogAgain = context.Blogs.Find(3);

С IQueryable такое не работает, запрос все равно будет, но EF вернет, вероятно, те же объекты что и после первого запроса.


Find - возвращает объект из контекста, без дополнительного запроса. Добавленные объекты, но не сохраненные в базе возвращаются
Query - делает запрос к базе, присоединяет объекты к контексту (если появились новые), возвращает объекты из контекста (Если уже были!). Добавленные объекты, но не сохраненные в базе не возвращаются


Insert or update pattern

        context.Entry(blog).State = blog.BlogId == 0 ?
                                   EntityState.Added :
                                   EntityState.Modified;

        context.SaveChanges();
        

Handling Concurrency Conflicts

Resolving optimistic concurrency exceptions with Reload (database wins)

using (var context = new BloggingContext())
{
    var blog = context.Blogs.Find(1);
    blog.Name = "The New ADO.NET Blog";

    bool saveFailed;
    do
    {
        saveFailed = false;

        try
        {
            context.SaveChanges();
        }
        catch (DbUpdateConcurrencyException ex)
        {
            saveFailed = true;

            // Update the values of the entity that failed to save from the store
            ex.Entries.Single().Reload();
        }

    } while (saveFailed);
}


Resolving optimistic concurrency exceptions as client wins

using (var context = new BloggingContext())
{
    var blog = context.Blogs.Find(1);
    blog.Name = "The New ADO.NET Blog";

    bool saveFailed;
    do
    {
        saveFailed = false;
        try
        {
            context.SaveChanges();
        }
        catch (DbUpdateConcurrencyException ex)
        {
            saveFailed = true;

            // Update original values from the database
            var entry = ex.Entries.Single();
            entry.OriginalValues.SetValues(entry.GetDatabaseValues());
        }

    } while (saveFailed);
}