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Object Relational Mapping (ORM)

When we first learned how to save data we started with reading and writing to files.

However, there are some significant issues with this process.

  1. We have to rewrite the entire file each time we add, change, or remove information. With a large amount of data this is burdensome.
  2. There is no way to relate information across files.
  3. There is not a fast way to access a single element of this information. We have to open and read as much of the file until we find the information.

For these reasons, and others, our more complex applications will require us to store information in a process that provides these features.

In another lesson we introduced the idea of Relational Databases and the SQL language. The relational database provides for all of the features we want in a data storage process.

  1. We can add, change, and remove information selectively.
  2. It provides a way to relate information across tables.
  3. We can directly, and quickly, look up specific information from any table.
  4. Most, if not all, databases adhere to the concept of ACID.
  • Atomic - updates happen as a single change. That is, we can change multiple rows, perhaps across multiple tables in one change. This helps ensure we do not have others read half of our update before we are complete.
  • Consistent - Each change we make keeps the database in a valid state. That is, all the rules (foreign keys, data types, validations) are always in effect and keep invalid data out of the system.
  • Isolation - Changes are processed concurrently (at the same time) however the state of the database would be the same if we processed the changes sequentially. This allows for us to ensure data safety while ensuring many users can update the system at the same time
  • Durability - Once a change has been recognized by the database it remains changed even in the case of a system failure (power loss, crash, etc.) That is, once the database tells us it has the data safely stored, we can believe it.

With all these features we would like to use a relational database in our systems. However, our systems are Object Oriented. We aren't coding in the landscape of tables, and rows, but lists and objects.

Enter the Object Relational Mapper (ORM)

Luckily we have a concept called an Object Relational Mapper (ORM). The purpose of an ORM is to seamlessly translate between the language of lists/objects to tables/rows. With an ORM we can continue to use all of our familiar C# ideas and allow the ORM to take care of the details of the appropriate SQL and relational database work.

------------
| |
| OUR CODE |
| |
------------
^
|
v
------------ ------------
| | | |
| ORM | <-----> | DATABASE |
| | | |
------------ ------------

Enter Entity Framework Core for .NET

In .NET Core we will be using the Entity Framework as our ORM of choice. This will allow us to setup relationships between our database tables and our C# classes, which we will refer to as POCOs -- Plain Old C-sharp Objects.

By introducing a few concepts we will be able to use the familiar LINQ statements to create, read, update, and delete rows of data. The Entity Framework Core (EF Core, or EF) has a number of classes and helper tools to make our lives easier. In this next section we will introduce the basics we need to get started.

Let's use an existing database

In our lesson on SQL Joins we had a database setup that looks like this:

+--------------------------------+ +---------------------------+
| Movies | | Ratings |
| | | |
| Id SERIAL | | Id SERIAL |
| Title TEXT | many one | Description TEXT |
| PrimaryDirector TEXT <-----------> |
| YearReleased INT | +---------------------------+
| Genre TEXT |
| |
+-------------^------------------+
| one
|
|
|
|
| many
+-------v----------------+ +-------------------------+
| Roles | | Actors |
| | many one | |
| Id SERIAL |<------------> | Id SERIAL |
| CharacterName TEXT | | FullName TEXT |
| | | Birthday DATE |
| | | |
+------------------------+ +-------------------------+

and we are eventually going to represent these relationships in our C# code.

If you have not read that lesson, or you have not generated a database from that lesson, and you intend on coding-along, stop now and create that database, tables, and data so that this lesson will work.

Creating our first EF Core application

Start by making a plain console application so we can prompt the user if needed as well as output information.

dotnet new sdg-console -o SuncoastMovies

we will also need to add the Entity Framework library to our project

cd SuncoastMovies
dotnet add package Npgsql.EntityFrameworkCore.PostgreSQL

Define our first Model

We will begin by accessing the list of movies from our database using our C# code.

Before we can do that we need to teach C# about the structure of a movie. We can do that by defining a class named Movie (singular) and give it properties corresponding to the same names we find in our table.

In our Program.cs we can add class Movie as such:

using System;
namespace SuncoastMovies
{
class Movie
{
public int Id { get; set; }
public string Title { get; set; }
public string PrimaryDirector { get; set; }
public int YearReleased { get; set; }
public string Genre { get; set; }
}
class Program
{
static void Main(string[] args)
{
}
}
}

Notice how we have modeled the property names and types to match the column names and types in the database. This is how EF Core will know how to MAP the information between our table and our objects. Here we are following a convention that allows us to keep the same ideas between our object world and our database world. We can, of course, use different names between our objects and our tables. Sometimes this is done if you are accessing an old database which used unfriendly names.

These classes are typically referred to as Models.

You may also notice that there isn't anything in this class that indicates something about the database. In order to make this link we need an additional class.

Connecting to the database through a context

To connect our POCO models to the database we use a Database Context, or DbContext for short. The purpose of the database context is to tell our code what models in the database are accessible as well as how to connect to the database.

Our database context looks like this:

// Define a database context for our Suncoast Movies database.
// It derives from (has a parent of) DbContext so we get all the
// abilities of a database context from EF Core.
class SuncoastMoviesContext : DbContext
{
// Define a movies property that is a DbSet.
public DbSet<Movie> Movies { get; set; }
// Define a method required by EF that will configure our connection
// to the database.
//
// DbContextOptionsBuilder is provided to us. We then tell that object
// we want to connect to a postgres database named suncoast_movies on
// our local machine.
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseNpgsql("server=localhost;database=SuncoastMovies");
}
}

NOTE, this will require using Microsoft.EntityFrameworkCore;

Let's walk through this class. It is derived from DbContext which is provided by EF Core. The DbContext is what allows us to connect to our database and relate our models to tables.

We then define a property Movies (plural) of type DbSet<Movie>. A DbSet will function similar to a List but have abilities to work with the data coming from the database. Specifically, we can use LINQ with a DbSet in the same ways we worked with LINQ and List. The Movie here is the model we wish to relate and Movies corresponds to the Movies table in our database. DbSet will act much like our List collection but has much more knowledge of how to read and write from the database.

Finally, we override a method required by EF Core that tells us how to connect to the database. EF Core will call this method to setup the connection to the database. In here we tell the options we are using a postgres database (UseNpgsql) and that the database is named suncoast_movies and is on our local machine.

Using the context to get at data

Here is our code so far:

using System;
using Microsoft.EntityFrameworkCore;
namespace SuncoastMovies
{
class Movie
{
public int Id { get; set; }
public string Title { get; set; }
public string PrimaryDirector { get; set; }
public int YearReleased { get; set; }
public string Genre { get; set; }
}
// Define a database context for our Suncoast Movies database.
// It derives from (has a parent of) DbContext so we get all the
// abilities of a database context from EF Core.
class SuncoastMoviesContext : DbContext
{
// Define a movies property that is a DbSet.
public DbSet<Movie> Movies { get; set; }
// Define a method required by EF that will configure our connection
// to the database.
//
// DbContextOptionsBuilder is provided to us. We then tell that object
// we want to connect to a postgres database named suncoast_movies on
// our local machine.
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseNpgsql("server=localhost;database=SuncoastMovies");
}
}
class Program
{
static void Main(string[] args)
{
}
}
}

Now we can add code to the Main to use our context and access our data.

// Get a new context which will connect to the database
var context = new SuncoastMoviesContext();

With this object we access our Movies property:

// Get a reference to our collection of movies.
// NOTE: this doesn't yet access any of them, just gives
// us a variable that knows how.
context.Movies;

Now with this movies object we can use some of our familiar LINQ capabilities. We start by seeing how we would count the number of movies in the table.

Counting movies

var movieCount = context.Movies.Count();
Console.WriteLine($"There are {movieCount} movies!");

NOTE: this will require using System.Linq;

This should seem familiar as the movies.Count() statement eventually becomes a SELECT COUNT(*) FROM MOVIES in our database. This is a perfect example of the idea of an ORM. At the level of C# we are working with a DbSet and the Count() method from LINQ, however, EF Core is translating this to the appropriate SQL statements and returning us the data we need.

Running this code we will see the output of:

There are 14 movies!

Getting a list of all the movies

To see all of the movies, we can use a foreach loop:

foreach (var movie in context.Movies)
{
Console.WriteLine($"There is a movie named {movie.Title}");
}

Again, translated to SQL this would be SELECT * FROM MOVIES. However, here we receive instances of our Movie class we can use to output information such as each movie object's title: movie.Title.

There are 14 movies!
There is a movie named Howls Moving Castle
There is a movie named Hitchhikers Guide to the Galaxy
There is a movie named The Lost World
There is a movie named Pirates of the Caribbean: The Curse of the Black Pearl
There is a movie named Harry Potter and Goblet of Fire
There is a movie named The Hobbit: An Unexpected Journey
There is a movie named The Hobbit: The Desolation of Smaug
There is a movie named The Hobbit: The Battle of the Five Armies
There is a movie named The Lord of the Rings: The Return of the King
There is a movie named The Lord of the Rings: The Fellowship of the Ring
There is a movie named The Lord of the Rings: The Two Towers
There is a movie named Cujo
There is a movie named It
There is a movie named It

So our application now looks like this:

using System;
using System.Linq;
using Microsoft.EntityFrameworkCore;
namespace SuncoastMovies
{
class Movie
{
public int Id { get; set; }
public string Title { get; set; }
public string PrimaryDirector { get; set; }
public int YearReleased { get; set; }
public string Genre { get; set; }
}
// Define a database context for our Suncoast Movies database.
// It derives from (has a parent of) DbContext so we get all the
// abilities of a database context from EF Core.
class SuncoastMoviesContext : DbContext
{
// Define a movies property that is a DbSet.
public DbSet<Movie> Movies { get; set; }
// Define a method required by EF that will configure our connection
// to the database.
//
// DbContextOptionsBuilder is provided to us. We then tell that object
// we want to connect to a postgres database named suncoast_movies on
// our local machine.
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseNpgsql("server=localhost;database=SuncoastMovies");
}
}
class Program
{
static void Main(string[] args)
{
var context = new SuncoastMoviesContext();
var movieCount = context.Movies.Count();
Console.WriteLine($"There are {movieCount} movies!");
foreach (var movie in context.Movies)
{
Console.WriteLine($"There is a movie named {movie.Title}");
}
}
}
}

Bonus: Seeing what SQL our code is generating.

It is often interesting to see the SQL being generated and executed by our application. To do this we need to configure some code to add a logger to our application. A logger is code that receives events from the system and then outputs a descriptive text describing the event. In this case, the interactions with the database.

Before we can use the logger we must add another package to our application:

dotnet add package Microsoft.Extensions.Logging.Console

Then we can add this code to our OnConfiguring method:

var loggerFactory = LoggerFactory.Create(builder => builder.AddConsole());
optionsBuilder.UseLoggerFactory(loggerFactory);

This will take any of the database events and output them to the console where we can see them.

For instance, when running the application the output now looks like:

info: Microsoft.EntityFrameworkCore.Infrastructure[10403]
Entity Framework Core 3.1.3 initialized 'SuncoastMoviesContext' using provider 'Npgsql.EntityFrameworkCore.PostgreSQL' with options: None
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
Executed DbCommand (9ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
SELECT COUNT(*)::INT
FROM movies AS m
There are 14 movies!
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
Executed DbCommand (22ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
SELECT m.id, m.genre, m.primary_director, m.title, m.year_released
FROM movies AS m
There is a movie named The Lost World
There is a movie named Pirates of the Caribbean: The Curse of the Black Pearl
There is a movie named Harry Potter and Goblet of Fire
There is a movie named The Hobbit: An Unexpected Journey
There is a movie named The Hobbit: The Desolation of Smaug
There is a movie named The Hobbit: The Battle of the Five Armies
There is a movie named The Lord of the Rings: The Return of the King
There is a movie named Cujo
There is a movie named It
There is a movie named It
There is a movie named Howls Moving Castle
There is a movie named The Lord of the Rings: The Fellowship of the Ring
There is a movie named The Lord of the Rings: The Two Towers
There is a movie named Hitchhikers Guide to the Galaxy

Adding in related tables.

Our database has other tables we could add to our system: Ratings, Roles, and Actors.

Let's add a model for our ratings.

class Rating
{
public int Id { get; set; }
public string Description { get; set; }
}
// Add this inside `SuncoastMoviesContext`
// Define a Ratings property that is a DbSet.
public DbSet<Rating> Ratings { get; set; }

Now we would be able to query our table of ratings. This is great, but what we want to do is see the rating for a specific movie. To achieve this we need to relate the two tables.

We need to modify the Movie model to tell it how to relate to the Rating model. To do this, we'll add the following code:

public int RatingId { get; set; }
public Rating Rating { get; set; }

This tells the Movie model that it can use the Rating property to return a Rating object.

Now when we access the context.Movies we can also tell it to fetch the related Rating via the Include method.

var moviesWithRatings = context.Movies.Include(movie => movie.Rating);

Now when we access the movies we are going to generate a JOIN to the ratings table and bring that information back again.

Now we can change our loop to also show the rating if it has one.

var moviesWithRatings = context.Movies.Include(movie => movie.Rating);
foreach (var movie in moviesWithRatings)
{
if (movie.Rating == null)
{
Console.WriteLine($"There is a movie named {movie.Title} and has not been rated yet");
}
else
{
Console.WriteLine($"There is a movie named {movie.Title} and a rating of {movie.Rating.Description}");
}
}

Notice how if there is no related Rating we need to see that the movie.Rating is not null. This will be the case for each row in Movies that doesn't have a RatingId that matches something from the Ratings table.

If we run this code we will see the SQL joins being generated:

info: Microsoft.EntityFrameworkCore.Infrastructure[10403]
Entity Framework Core 3.1.3 initialized 'SuncoastMoviesContext' using provider 'Npgsql.EntityFrameworkCore.PostgreSQL'
with options: None
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
Executed DbCommand (15ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
SELECT COUNT(*)::INT
FROM "Movies" AS m
There are 14 movies!
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
Executed DbCommand (1ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
SELECT m."Id", m."Genre", m."PrimaryDirector", m."RatingId", m."Title", m."YearReleased", r."Id", r."Description"
FROM "Movies" AS m
INNER JOIN "Ratings" AS r ON m."RatingId" = r."Id"
There is a movie named Hitchhikers Guide to the Galaxy and a rating of PG
There is a movie named The Lost World and a rating of PG-13
There is a movie named Pirates of the Caribbean: The Curse of the Black Pearl and a rating of PG-13
There is a movie named Harry Potter and Goblet of Fire and a rating of PG-13
There is a movie named The Hobbit: An Unexpected Journey and a rating of PG-13
There is a movie named The Hobbit: The Desolation of Smaug and a rating of PG-13
There is a movie named The Hobbit: The Battle of the Five Armies and a rating of PG-13
There is a movie named The Lord of the Rings: The Return of the King and a rating of PG-13
There is a movie named The Lord of the Rings: The Fellowship of the Ring and a rating of PG-13
There is a movie named The Lord of the Rings: The Two Towers and a rating of PG-13
There is a movie named Cujo and a rating of R
There is a movie named It and a rating of R
There is a movie named It and a rating of R

Here is our code so far:

using System;
using System.Linq;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
namespace SuncoastMovies
{
class Movie
{
public int Id { get; set; }
public string Title { get; set; }
public string PrimaryDirector { get; set; }
public int YearReleased { get; set; }
public string Genre { get; set; }
public int RatingId { get; set; }
public Rating Rating { get; set; }
}
class Rating
{
public int Id { get; set; }
public string Description { get; set; }
}
// Define a database context for our Suncoast Movies database.
// It derives from (has a parent of) DbContext so we get all the
// abilities of a database context from EF Core.
class SuncoastMoviesContext : DbContext
{
// Define a movies property that is a DbSet.
public DbSet<Movie> Movies { get; set; }
// Define a Ratings property that is a DbSet.
public DbSet<Rating> Ratings { get; set; }
// Define a method required by EF that will configure our connection
// to the database.
//
// DbContextOptionsBuilder is provided to us. We then tell that object
// we want to connect to a postgres database named suncoast_movies on
// our local machine.
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
var loggerFactory = LoggerFactory.Create(builder => builder.AddConsole());
optionsBuilder.UseLoggerFactory(loggerFactory);
optionsBuilder.UseNpgsql("server=localhost;database=SuncoastMovies");
}
}
class Program
{
static void Main(string[] args)
{
var context = new SuncoastMoviesContext();
var movieCount = context.Movies.Count();
Console.WriteLine($"There are {movieCount} movies!");
var moviesWithRatings = context.Movies.Include(movie => movie.Rating);
foreach (var movie in moviesWithRatings)
{
if (movie.Rating == null)
{
Console.WriteLine($"There is a movie named {movie.Title} and has not been rated yet");
}
else
{
Console.WriteLine($"There is a movie named {movie.Title} and a rating of {movie.Rating.Description}");
}
}
}
}
}

Add in other relationships.

Finally, we can add the relationships for Role and Actor.

Notice that when adding the Roles relationship to the Movie class we add it as such:

public List<Role> Roles { get; set; }

We are telling the Movie that it has a list of related Roles. Since we have setup our relationships with appropriate SQL syntax, EF Core can determine how to join these tables together.

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
namespace SuncoastMovies
{
class Movie
{
public int Id { get; set; }
public string Title { get; set; }
public string PrimaryDirector { get; set; }
public int YearReleased { get; set; }
public string Genre { get; set; }
// This is the column in the database
public int RatingId { get; set; }
// This is the related object we can use from our code (if properly used with Include)
public Rating Rating { get; set; }
// This is the related list of roles we an use (if properly used with Include)
public List<Role> Roles { get; set; }
}
class Rating
{
public int Id { get; set; }
public string Description { get; set; }
}
class Role
{
public int Id { get; set; }
public string CharacterName { get; set; }
// This is the column in the database
public int MovieId { get; set; }
// This is the related object we can use from our code (if properly used with Include)
public Movie Movie { get; set; }
// This is the column in the database
public int ActorId { get; set; }
// This is the related object we can use from our code (if properly used with Include)
public Actor Actor { get; set; }
}
class Actor
{
public int Id { get; set; }
public string FullName { get; set; }
public DateTime Birthday { get; set; }
// This is the related list of roles we an use (if properly used with Include)
public List<Role> Roles { get; set; }
}
// Define a database context for our Suncoast Movies database.
// It derives from (has a parent of) DbContext so we get all the
// abilities of a database context from EF Core.
class SuncoastMoviesContext : DbContext
{
// Define a movies property that is a DbSet.
public DbSet<Movie> Movies { get; set; }
// Define a Ratings property that is a DbSet.
public DbSet<Rating> Ratings { get; set; }
// Define a Roles property that is a DbSet.
public DbSet<Role> Roles { get; set; }
// Define an Actors property that is a DbSet.
public DbSet<Actor> Actors { get; set; }
// Define a method required by EF that will configure our connection
// to the database.
//
// DbContextOptionsBuilder is provided to us. We then tell that object
// we want to connect to a postgres database named suncoast_movies on
// our local machine.
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
var loggerFactory = LoggerFactory.Create(builder => builder.AddConsole());
optionsBuilder.UseLoggerFactory(loggerFactory);
optionsBuilder.UseNpgsql("server=localhost;database=SuncoastMovies");
}
}
class Program
{
static void Main(string[] args)
{
var context = new SuncoastMoviesContext();
var movieCount = context.Movies.Count();
Console.WriteLine($"There are {movieCount} movies!");
// Makes a new collection of movies but each movie knows the associated Rating object
var moviesWithRatingsRolesAndActors = context.Movies.
// from our movie, please include the associated Rating object
Include(movie => movie.Rating).
// ... and from our movie, please include the associated Roles LIST
Include(movie => movie.Roles).
// THEN for each of roles, please include the associated Actor object
ThenInclude(role => role.Actor);
foreach (var movie in moviesWithRatingsRolesAndActors)
{
if (movie.Rating == null)
{
Console.WriteLine($"There is a movie named {movie.Title} and has not been rated yet");
}
else
{
Console.WriteLine($"There is a movie named {movie.Title} and a rating of {movie.Rating.Description}");
}
foreach (var role in movie.Roles)
{
Console.WriteLine($" - Has a character named {role.CharacterName} played by {role.Actor.FullName}");
}
}
}
}
}

And our program's output is now.

info: Microsoft.EntityFrameworkCore.Infrastructure[10403]
Entity Framework Core 3.1.3 initialized 'SuncoastMoviesContext' using provider 'Npgsql.EntityFrameworkCore.PostgreSQL' with options: None
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
Executed DbCommand (22ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
SELECT COUNT(*)::INT
FROM "Movies" AS m
There are 14 movies!
info: Microsoft.EntityFrameworkCore.Database.Command[20101]
Executed DbCommand (2ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
SELECT m."Id", m."Genre", m."PrimaryDirector", m."RatingId", m."Title", m."YearReleased", r."Id", r."Description", t."Id", t."ActorId", t."CharacterName", t."MovieId", t."Id0", t."Birthday", t."FullName"
FROM "Movies" AS m
INNER JOIN "Ratings" AS r ON m."RatingId" = r."Id"
LEFT JOIN (
SELECT r0."Id", r0."ActorId", r0."CharacterName", r0."MovieId", a."Id" AS "Id0", a."Birthday", a."FullName"
FROM "Roles" AS r0
INNER JOIN "Actors" AS a ON r0."ActorId" = a."Id"
) AS t ON m."Id" = t."MovieId"
ORDER BY m."Id", r."Id", t."Id", t."Id0"
There is a movie named The Lost World and a rating of PG-13
There is a movie named Pirates of the Caribbean: The Curse of the Black Pearl and a rating of PG-13
- Has a character named Will Turner played by Orlando Bloom
There is a movie named Harry Potter and Goblet of Fire and a rating of PG-13
- Has a character named Filius Flitwick played by Warwick Davis
There is a movie named The Hobbit: An Unexpected Journey and a rating of PG-13
- Has a character named Bilbo played by Martin Freeman
There is a movie named The Hobbit: The Desolation of Smaug and a rating of PG-13
- Has a character named Bilbo played by Martin Freeman
There is a movie named The Hobbit: The Battle of the Five Armies and a rating of PG-13
- Has a character named Bilbo played by Martin Freeman
There is a movie named The Lord of the Rings: The Return of the King and a rating of PG-13
- Has a character named Legolas played by Orlando Bloom
There is a movie named The Lord of the Rings: The Fellowship of the Ring and a rating of PG-13
- Has a character named Legolas played by Orlando Bloom
There is a movie named The Lord of the Rings: The Two Towers and a rating of PG-13
- Has a character named Legolas played by Orlando Bloom
There is a movie named Hitchhikers Guide to the Galaxy and a rating of PG
- Has a character named Marvin played by Warwick Davis
- Has a character named Arthur Dent played by Martin Freeman
There is a movie named Cujo and a rating of R
There is a movie named It and a rating of R
There is a movie named It and a rating of R

Adding data to our database.

To add a Movie to the record we can make a new POCO:

var newMovie = new Movie {
Title = "SpaceBalls",
PrimaryDirector = "Mel Brooks",
Genre = "Comedy",
YearReleased = 1987,
RatingId = 2
};

Then we can add this POCO to the Movies context and tell the context to save the changes

context.Movies.Add(newMovie);
context.SaveChanges();

After adding this code and entering some sample data we would see this query:

INSERT INTO "Movies" ("Genre", "PrimaryDirector", "RatingId", "Title", "YearReleased")
VALUES (@p0, @p1, @p2, @p3, @p4)
RETURNING "Id";

Updating a movie

To change the name of a movie, we can find an existing record first before changing an attribute.

// Search for a movie by name. FirstOrDefault takes a function to use to compare the movies and returns the first record that matches, or if nothing matches, returns null.
// This is the same as we used with LINQ against a List, but this time it is searching the database.
var existingMovie = context.Movies.FirstOrDefault(movie => movie.Title == "SpaceBalls");
// If we found an existing movie.
if (existingMovie != null) {
// Change the title of this movie.
existingMovie.Title = "SpaceBalls - the best movie ever";
// Ask the context to save changes.
context.SaveChanges();
}

Deleting a movie

To remove a movie we find the movie and then remove it.

var existingMovie = context.Movies.FirstOrDefault(movie => movie.Title == "Cujo");
// If we found an existing movie.
if (existingMovie != null) {
// Remove the existing movie from the collection
context.Movies.Remove(existingMovie);
// Ask the context to save changes, in this case deleting the record.
context.SaveChanges();
}

A whole app that can create, read, update, and delete movies:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
namespace SuncoastMovies
{
class Movie
{
public int Id { get; set; }
public string Title { get; set; }
public string PrimaryDirector { get; set; }
public int YearReleased { get; set; }
public string Genre { get; set; }
// This is the column in the database
public int RatingId { get; set; }
// This is the related object we can use from our code (if properly used with Include)
public Rating Rating { get; set; }
// This is the related list of roles we an use (if properly used with Include)
public List<Role> Roles { get; set; }
}
class Rating
{
public int Id { get; set; }
public string Description { get; set; }
}
class Role
{
public int Id { get; set; }
public string CharacterName { get; set; }
// This is the column in the database
public int MovieId { get; set; }
// This is the related object we can use from our code (if properly used with Include)
public Movie Movie { get; set; }
// This is the column in the database
public int ActorId { get; set; }
// This is the related object we can use from our code (if properly used with Include)
public Actor Actor { get; set; }
}
class Actor
{
public int Id { get; set; }
public string FullName { get; set; }
public DateTime Birthday { get; set; }
// This is the related list of roles we an use (if properly used with Include)
public List<Role> Roles { get; set; }
}
// Define a database context for our Suncoast Movies database.
// It derives from (has a parent of) DbContext so we get all the
// abilities of a database context from EF Core.
class SuncoastMoviesContext : DbContext
{
// Define a movies property that is a DbSet.
public DbSet<Movie> Movies { get; set; }
// Define a Ratings property that is a DbSet.
public DbSet<Rating> Ratings { get; set; }
// Define a Roles property that is a DbSet.
public DbSet<Role> Roles { get; set; }
// Define an Actors property that is a DbSet.
public DbSet<Actor> Actors { get; set; }
// Define a method required by EF that will configure our connection
// to the database.
//
// DbContextOptionsBuilder is provided to us. We then tell that object
// we want to connect to a postgres database named suncoast_movies on
// our local machine.
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
var loggerFactory = LoggerFactory.Create(builder => builder.AddConsole());
optionsBuilder.UseLoggerFactory(loggerFactory);
optionsBuilder.UseNpgsql("server=localhost;database=SuncoastMovies");
}
}
class Program
{
static void Main(string[] args)
{
var context = new SuncoastMoviesContext();
var keepGoing = true;
while (keepGoing)
{
Console.Write("(L)ist movies. (C)reate movie. (D)elete movie. (U)pdate movie. (Q)uit: ");
var option = Console.ReadLine().ToUpper();
switch (option)
{
case "Q":
keepGoing = false;
break;
case "L":
var movies = context.Movies.Include(movie => movie.Rating).Include(movie => movie.Roles).ThenInclude(role => role.Actor);
var movieCount = movies.Count();
Console.WriteLine($"There are {movieCount} movies!");
foreach (var movie in movies)
{
if (movie.Rating == null)
{
Console.WriteLine($"There is a movie named {movie.Title} and has not been rated yet");
}
else
{
Console.WriteLine($"There is a movie named {movie.Title} and a rating of {movie.Rating.Description}");
}
foreach (var role in movie.Roles)
{
Console.WriteLine($" - Has a character named {role.CharacterName} played by {role.Actor.FullName}");
}
}
break;
case "C":
Console.Write("What is the name of the new movie: ");
var title = Console.ReadLine();
Console.Write("What is the name of the primary director: ");
var primaryDirector = Console.ReadLine();
Console.Write("What is the genre: ");
var genre = Console.ReadLine();
Console.Write("In what year was the movie released: ");
var yearReleased = int.Parse(Console.ReadLine());
Console.Write("What is the ID of the movie rating: ");
var ratingID = int.Parse(Console.ReadLine());
var newMovie = new Movie
{
Title = title,
PrimaryDirector = primaryDirector,
Genre = genre,
YearReleased = yearReleased,
RatingId = ratingID
};
context.Movies.Add(newMovie);
context.SaveChanges();
break;
case "U":
Console.Write("What is the name of the movie you want to update: ");
var titleOfMovieToUpdate = Console.ReadLine();
var existingMovieToUpdate = context.Movies.FirstOrDefault(movie => movie.Title == titleOfMovieToUpdate);
if (existingMovieToUpdate != null)
{
Console.Write("Enter a new name for the movie: ");
existingMovieToUpdate.Title = Console.ReadLine();
context.SaveChanges();
}
else
{
Console.WriteLine($"No movie with title {titleOfMovieToUpdate} to update");
}
break;
case "D":
Console.Write("What is the name of the movie you want to delete: ");
var titleOfMovieToDelete = Console.ReadLine();
var existingMovieToDelete = context.Movies.FirstOrDefault(movie => movie.Title == titleOfMovieToDelete);
if (existingMovieToDelete != null)
{
context.Movies.Remove(existingMovieToDelete);
context.SaveChanges();
}
else
{
Console.WriteLine($"No movie with title {titleOfMovieToDelete} to delete");
}
break;
}
}
}
}
}
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