Entity Framework 5 on PostgreSQL

This walkthrough will get you started with an application that uses the Entity Framework (EF) to read and write data from a PostgreSQL database. It is intended to be similar to the Code First to a New Database walkthrough.

There are currently two PostgreSQL providers for EF that I know of: Npgsql and Devart's dotConnect for PostgreSQL. Devart's provider has a much richer set of features, but it is also a commercial product. In the spirit of FOSS, we will be using the Npgsql provider for this walkthrough. I encourage you to keep the Devart provider in mind, however, if your project requires that extra level of support.

Create the Application

For simplicity, we will be using a Console Application, but the basic steps are the same regardless of project type.

1. Open Visual Studio
2. Select File -> New -> Project...
3. Select Console Application
4. Name the project
5. Click OK

Create the Model

For our model, we'll be borrowing pieces from the Chinook Database (a cross-platform, sample database). Specifically, we will be using Artists and Albums.

Add the following two classes to your project.

public class Artist
{
    public Artist()
    {
        Albums = new List<Album>();
    }

    public int ArtistId { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Album> Albums { get; set; }
}

public class Album
{
    public int AlbumId { get; set; }
    public string Title { get; set; }

    public int ArtistId { get; set; }
    public virtual Artist Artist { get; set; }
}

Create a Context

In EF, the context becomes your main entry point into the database. Before we define our context though, we will need to install Entity Framework.

1. Select Tools -> Library Package Manager -> Package Manager Console
2. Inside the Package Manager Console (PMC) run Install-Package EntityFramework

Now, add the context class to your project.

class ChinookContext : DbContext
{
    public DbSet<Artist> Artists { get; set; }
    public DbSet<Album> Albums { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        // Map to the correct Chinook Database tables
        modelBuilder.Entity<Artist>().ToTable("Artist", "public");
        modelBuilder.Entity<Album>().ToTable("Album", "public");

        // Chinook Database for PostgreSQL doesn't auto-increment Ids
        modelBuilder.Conventions
            .Remove<StoreGeneratedIdentityKeyConvention>();
    }
}

Install the Provider

In order to connect to PostgreSQL databases, we will need to install an appropriate ADO.NET and Entity Framework provider. Luckily, the provider we're using is available via NuGet.

1. Inside PMC, run Install-Package Npgsql

We also need to register the provider. Open App.config, and anywhere inside the configuration element, add the following fragment.

<system.data>
  <DbProviderFactories>
    <add name="Npgsql Data Provider"
          invariant="Npgsql"
          description="Data Provider for PostgreSQL"
          type="Npgsql.NpgsqlFactory, Npgsql" />
  </DbProviderFactories>
</system.data>

Add the Database

Unfortunately, Npgsql does not support creating databases. So instead of letting Code First create our database, we will need to manually add a database to our project. It's a good thing we're using a cross-platform sample database!

When I started writing this, Chinook Database actually wasn't available for PostgreSQL. Fortunately, it's an open source project so I've submitted a pull request that adds support for PostgreSQL. It was released as part of version 1.4.

1. Download and extract the PostgreSQL version of the Chinook Database
2. Run CreatePostgreSql.bat

Also, add a connection string to the App.Config that points to the database. Anywhere inside the configuration element, add the following fragment.

<connectionStrings>
  <add name="ChinookContext"
        connectionString=
"Server=localhost;Database=chinook;User Id=postgres;Password=P4ssw0rd;"
        providerName="Npgsql" />
</connectionStrings>

Start Coding

Ok, we should be ready to start coding our application. Let's see what artists exist in the database. Inside Program.cs, add the following to Main.

using (var db = new ChinookContext())
{
    var artists = from a in db.Artists
                  where a.Name.StartsWith("A")
                  orderby a.Name
                  select a;

    foreach (var artist in artists)
    {
        Console.WriteLine(artist.Name);
    }
}

Hmm, it looks like one of my favorite bands is missing. Let's add it.

using (var db= new ChinookContext())
{
    db.Artists.Add(
        new Artist
        {
            ArtistId = 276,
            Name = "Anberlin",
            Albums =
            {
                new Album
                {
                    AlbumId = 348,
                    Title = "Cities"
                },
                new Album
                {
                    AlbumId = 349,
                    Title = "New Surrender"
                }
            }
        });
    db.SaveChanges();
}

We can also update and delete existing data like this.

using (var context = new ChinookContext())
{
    var police = db.Artists.Single(a => a.Name == "The Police");
    police.Name = "Police, The";

    var avril = db.Artists.Single(a => a.Name == "Avril Lavigne");
    context.Artists.Remove(avril);

    db.SaveChanges();
}

Conclusion

Hopefully by now, you have enough information to get started using the Entity Framework with a PostgreSQL database. For many, many more articles on how to use EF, check out our team's official Getting Started page on MSDN.