DbContext

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DbContext
The DbContext is simply the way for the developers to incorporate Entity Framework based data to the application.
It allows you to make database connections inside an application model and allows the developer to link the model properties to the database table using a connection string.
It is the base class to manage all types of database operations, such as establishing a connection with the database, query the database and end the connection.
In the older version of Entity Framework, there was an ObjectContext class, but now in the latest version, we have DbContext class.
The DbContext in Entity Framework Core consist of the following features and responsibilities:
Database Management
Database Connections
Entity Set
Querying
Change Tracking
Persisting Data
Caching
Model Binding
Materialization
Configuration
Validation
All these features are present to perform a dedicated task on the Entity Framework Core.
Database Management
In Entity Framework Core, the DbContext allows the user to manage the complete database. It allows the user to create, delete or check for the existing database connections inside the given project.
Database Connections
The DbContext also allows the user to check, establish and close the connections between the databases as per the requirement of the project.
Entity Set
The DbContext represents all the entities in a project, it also manages these entities throughout their lifetime. It also applies CRUD operations on all the entity types, such as Add, Attach or Remove.
Querying
The DbContext also converts the LINQ queries to the SQL queries using the querying method.
Change Tracking
In Entity Framework, the DbContext includes the Change Tracker API which tracks all the changes when entities are added, updated or deleted.
The state of the entity can be changed manually or automatically depending on the user.
Persisting Data
Since DbContext performs all the CRUD related operations, It persists all the changes made to the database.
Caching
The DbContext keeps all the changes made to the entities throughout their lifetime in the form of first-level cache files.
Model Binding
The DbContext also performs model binding operations in which it automatically read classes and code-based configuration to build an in-memory model, metadata and database.
Materialization
In materialization, the DbContext converts the queries from the database tables into entities.
Configuration
The DbContext configures the behavior of the context of the databases.
Validations
The DbContext checks the validity of the data and performs automatic validation of the data.
DbContext Query
Using DbContext in Entity Framework Core, there are three types of query operations which can be performed, they are:
Adding a new entity.
Changing or modifying the properties of the current entity.
Deleting or removing the existing Entity.
Adding new Entity
To add a new entity using DbContext, the developer must prepare the relevant code related to the database.
This code is very straightforward and new entities can be added by using these four key methods:
Add<TEntity>
// with type parameter
var Actor = new Actor{ FirstName = "Dwayne", LastName = "Johnson" };
context.Add<Actor>(actor);
context.SaveChanges();
​
// without type parameter
var Actor = new Actor{ FirstName = "Dwayne", LastName = "Johnson" };
context.Add(actor);
context.SaveChanges();
Add(object entity)
object actor = new Actor{ FirstName = "Dwayne", LastName = "Johnson" };
context.Add(actor);
context.SaveChanges();
AddRange(IEnumerable<object> entities)
var context = new SampleContext();
var actor = new Actor()
{
    FirstName = "Dwayne",
    LastName = "Johnson",
    Movies = new List<Movie>()
    {
        new Movie { Title = "Baywatch"},
        new Movie { Title = "Rampage" },
        new Movie { Title = "SkyScraper" }
    }
};
​
context.Add(actor);
context.SaveChanges();
AddRange(params object [] entities)
var actor = new Actor{ FirstName = "Dwayne", LastName = "Johnson" };
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.Add(actor);
context.SaveChanges();
To add multiple records the developer must use the AddRange entity method.
Sample 1:
var context = new SampleContext();
var actor = new Actor()
{
    FirstName = "Dwayne",
    LastName = "Johnson",
    Movies = new List<Movie>()
    {
        new Movie { Title = "Baywatch"},
        new Movie { Title = "Rampage" },
        new Movie { Title = "SkyScraper" }
    }
};
​
context.AddRange(Movies);
context.SaveChanges();
Sample 2:
var context = new SampleContext();
var actor = new Actor{ FirstName = "Dwayne", LastName = "Johnson" };
var movie = new Movie { Title = "Baywatch", Actor= actor};
context.AddRange(actor, movie);
context.SaveChanges();
The SaveChanges method insert all entities to the database.
Changing/Modifying Entity
To change or modify an entity, the DbContext should track the current modifications of the entities, so whenever there will be a change in the entity, the DbContext will register it as Modified, and the change tracker API will record the current modifications and will also keep the previous changes.
There are various method for Modifying Entities, which are:
Disconnected Scenario.
var actor = context.Actors.First(a => a.ActorId == 1);
actor.FirstName = "Dwayne";
context.SaveChanges();
Setting Entitystate.
public void Save(Actor actor)
{
    context.Entry(actor).State = EntityState.Modified;
    context.SaveChanges();
}
DbContext Update.
public void Save(Actor actor)
{
    context.Update(actor);
    context.SaveChanges();
}
Attach.
var context = new TestContext();
var actor = new Actor()
{
    ActorId = 1,
    FirstName = "Dwayne",
    LastName = "Johnson"
};
​
actor.Movies.Add(new Movie {MovieId = 1, Title = "Baywatch" });
context.Attach(actor);
context.Entry(actor).Property("FirstName").IsModified = true;
context.SaveChanges();
TrackGraph.
Sample 1:
var actor = new Actor() 
{
    ActorId = 1,
    FirstName = "Dwayne",
    LastName = "Johnson"
};
​
actor.Movies.Add(new Movie { ActorId = 1, MovieId = 1, Title = "Baywatch", Isbn = "0123" });
actor.Movies.Add(new Movie { ActorId = 1, MovieId = 2, Title = "Rampage", Isbn = "0123" });
actor.Movies.Add(new Movie { ActorId = 1, MovieId = 3, Title = "SkyScraper", Isbn = "0123" });
​
var context = new TestContext();
context.ChangeTracker.TrackGraph(actor, e => 
{
    if((e.Entry.Entity as Actor) != null)
    {
        e.Entry.State = EntityState.Unchanged;
    }
    else
    {
        e.Entry.State = EntityState.Modified;
    }
});
​
context.SaveChanges();
Sample 2:
var actor = new Actor() 
{
    ActorId = 1,
    FirstName = "Dwayne",
    LastName = "Johnson"
};
​
actor.Movies.Add(new Movie { MovieId = 1, Title = "Baywatch", Isbn = "0123" });
actor.Movies.Add(new Movie { MovieId = 2, Title = "Rampage", Isbn = "0123" });
actor.Movies.Add(new Movie { MovieId = 3, Title = "SkyScraper", Isbn = "0123" });
​
var context = new TestContext();
​
context.ChangeTracker.TrackGraph(actor, e => 
{
    e.Entry.State = EntityState.Unchanged; //starts tracking
    if((e.Entry.Entity as Movie) != null)
    {
        context.Entry(e.Entry.Entity as Movie).Property("Isbn").IsModified = true;
    }
});
Deleting/Removing Entity
To delete an existing entity, the current entity should be continuously tracked by the DbContext in order to check its state and change it to Deleted.
To apply this approach, we use DbContext.Remove method, after applying this code, the DbContext usually executes two SQL Statements.
The first one to retrieve the entity from the database and the other one to delete it permanently from the Database.
To remove entities there are usually two properties:
Setting Entity State.
var context = new SampleContext();
var actor = new Actor { ActorId = 1 };
context.Entry(actor).State = EntityState.Deleted;
context.SaveChanges();
Related Data.
var context = new SampleContext();
var actor = context.Actors.Single(a => a.ActorId == 1);
var movies = context.Movies.Where(b => EF.Property<int>(b, "ActorId") == 1);
​
foreach (var movie in movies)
{
    actor.Movies.Remove(movie);
}
​
context.Remove(actor);
context.SaveChanges();
Entity Framework Core Model
DbSet
Last modified 1yr ago