DbSet

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DbSet
In Entity Framework Core, the DbSet represents the set of entities. In a database, a group of similar entities is called an Entity Set.
The DbSet enables the user to perform various operations like add, remove, update, etc. on the entity set.
Each entity type shows some DbSet properties to participate in CRUD operations.
In the working model, the DbContext represents the DbSet property of all the entities, and it keeps the collection of entities in memory.
Operations of DbSet
The DbSet is responsible for performing all the basic CRUD (Create, Read, Update and Delete) operations on each of the Entity.
The DbSet operations are used to change any property of the entity in the EF Core. The most essential methods of the DbSet are:
Querying Data
Adding Data
Modifying Data
Deleting Data
Querying of Data
In Entity Framework, querying a data is performed using DbSet and the queries are specified using LINQ.
To write LINQ queries, the .NET developers can use query syntax or method syntax.
The query syntax is similar to the SQL and EF Core provider is responsible to check if the query is converted to SQL or not so that the database can perform the execution of the code.
The method syntax is a chained method in which some of the queries are like the SQL query, but most of them are not.
In Entity Framework Core, there are various methods for querying the different type of data such as:
Retrieving Single Object
Retrieving Multiple objects
Filtering and Ordering
Grouping
Returning Non-Entity types
Include Related Data
NoTracking Queries
Retrieving Single object
To retrieve a single entity from a query, you can use First, FirstOrdefault, Single, SingleOrDefault and Find methods.
In the First and FirstOrDefault criteria, there is only one entity returned from a large group of entities.
Whereas in Single and SingleOrDefault one record entity is returned but that entity should be relevant to the required field otherwise it will be discarded.
The Find method is familiar to the users of the older version of Entity Framework that use to support the DbSet API.
The Find method requires a Key-value parameter in the entity to determine the required entity otherwise it gives null as the result.
var actor = context.Actors.First();
var actor = context.Actors.Where(a => a.ActorId == 1).Single();
var actor = context.Actors.Single(a => a.ActorId == 1);
var actor = context.Actors.Find(1);
Retrieving Multiple Objects
The queries that are retrieving the data which is compatible with multiple entities are executed otherwise those queries are ignored.
Data is iterated when the required query is executed in foreach loop, ToList, Sum or count. The query is not executed until the foreach loop is utilized.
Sample 1:
var movies = context.Movies; // define query
foreach(var movie in movies) // query executed and data obtained from database
{
    ...
}
Sample 2:
var movies = context.Movies.ToList(); // define query and force execution
Filtering and Ordering
Filtering is the process of picking up the correct entity using the where method, which has become the principle of the filtering process.
Sample 1:
var movies = context.Movies.Where(p => p.CategoryId == 1); // where method
Sample 2:
// lambda as expression that returns boolean
var movies = context.Movies.Where(p => p.CategoryId == 1 && p.Ratings< 5);
Sample 3:
var movies = context.Movies.OrderBy(p => p.MovieName);
var categories = context.Categories.OrderBy(c => c.CategoryName)
    .ThenOrderBy(c => c.CategoryId);
Grouping
It uses the GroupBy method which is used to group all the entity results according to their categories.
var groups = context.Movies.GroupBy(p => p.CategoryId);
​
var groups = context.Movies
    .GroupBy(p => new {Director = p.DirectorId, Country = p.CountryId});
Returning Non-Entity types
When the developer wants to keep the main data but needs to return the sub-data then Non-Entity types DbSet can be used.
The returned data can be stated as a non-entity type or the anonymous type, to perform this operation, query types can also be used as an alternate method.
public class MovieHeader
{
    public int MovieId { get; set; }
    public string MovieName { get; set; }
}
List<MovieHeader> headers = context.Movies.Select(p => new MovieHeader()
{
    MovieId = p.MovieId,
    MovieName = p.MovieName
}).ToList();
Include Related Data
Just as the name suggests the Include property is used to include the related entity type into the database.
Sample 1:
var actors = context.Actors.Include(a => a.Movies).ToList();
Sample 2:
var actors = context.Actors
    .Include(a => a.Producer)
    .Include(a => a.Movies)
    .ToList();
Sample 3:
var actors = context.Actors
    .Include(a => a.Movies)
    .ThenInclude(b => b.Producer)
    .ToList();
No-Tracking Queries
To improve the performance of the application the no-tracking queries are used in the form AsNoTracking method.
This will inform the DbContext that the entity is read-only and does not need to be tracked by the context, this will eventually decrease the load on the system, hence improving the performance.
var movies = context.Movies.AsNoTracking().ToList();
using (var context = new SampleContext())
{
    context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;
    var movies = context.Movies.ToList();
    var actors = context.Actors.ToList();
    …
}
Add Data
To add a new entity to the database, the Add method is used and then after tracking the entity by DbContext the state of the entity is changed to Added.
To add multiple records into the database, the AddRange method is used, it is like the Add method but in AddRange method, all the records are saved in just a single step instead of adding them all individually.
To add an entity using the DbSet there are 3 key methods:
Add (Entity)
// with type parameter
var actor = new Actor{ FirstName = "John", LastName = "Dwayne" };
context.Actors.Add<Actor>(actor);
​
// without type parameter
var actor = new Actor{ FirstName = "John", LastName = "Dwayne" };
context.Actors.Add(actor);
AddRange <Entity>
var context = new SampleContext();
​
var actor = new Actor() 
{
    FirstName = "John",
    LastName = "Dwayne" };
    var movies = new List<Movie>()
    {
        new Movie { Title = "Baywatch", Actor = actor},
        new Movie { Title = "Rampage", Actor = actor },
        new Movie { Title = "Skyscraper", Actor = actor }
    }
};
​
context.Movies.AddRange(movies);
Context.SaveChanges();
AddRange [Entity]
var actor = new Actor { FirstName = "John", LastName = "Dwayne" };
var baywatch = new Movie { Title = "Baywatch", Actor = actor };
var rampage = new Movie { Title = "Rampage", Actor = actor };
var skyscraper = new Movie { Title = "Skyscraper", Actor = actor };
​
context.Movies.AddRange(actor, baywatch, rampage, skyscraper);
context.SaveChanges();
Modify Data
To modify the data in DbSet, first it checks whether the data which is to be modified is being tracked or not.
After the Change tracker detects a change in the state of the entity, it issues a SQL statement that updates the properties that were changed.
var actor = context.Actors.Find(1);
actor.FirstName = "John R";
context.SaveChanges();
Disconnected Scenario in Modifying Data
If the application such as ASP.NET is disconnected from the DbContext, then the modifications must be updated using another method.
To update the state of the entity in disconnected form, the DbSet<T>.Update method is used, so whenever the application is loaded, it changes the state of the entity as Updated.
This method is newly introduced in Entity Framework Core.
DbSet Update
The Update method is using DbSet<T> class and provides different methods to work with an individual or multiple entities.
public void Save(Actor actor)
{
    context.Actors.Update(actor);
    context.SaveChanges();
}
Delete Data
Deleting the data is similar to modifying the data using DbSet. For deleting any data, it will first check whether the data which is to be deleted is being tracked by the context or not.
To determine the state of the entity DbSet uses DbSet<T>.Remove method which sets the state of the entity as Deleted, after this when the changes are saved, the DELETE statement is generated and executed by the Database.
Sample 1:
context.Actors.Remove(context.Actors.Find(1));
context.SaveChanges();
Sample 2:
var context = new SampleContext();
var actor = new Actor { ActorId = 1 };
context.Actors.Remove(actor);
context.SaveChanges();
Related Data in Deleting the in DbSet
If the entity that user wants to delete has a related data then the approach that you take will depend on how the relationship has been configured.
If the relation is a fully defined relationship, then the resulting output will be either deleted or set to null.
In other cases, EF Core has also introduced a Shadow property to represent the foreign key, this method is a little longer as it requires four action calls from the database.
var context = new SampleContext();
var actor = context.Actors.Find(1);
​
var movies = context.Movies.Where(b => EF.Property<int>(b, "ActorId") == 1);
foreach (var movie in movies)
{
    actor.Movies.Remove(movie);
}
​
context.Actors.Remove(actor);
context.SaveChanges();
DbContext
Relationship in EF-Core
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DbSet
Operations of DbSet