Override Mapping

EFCore handles target type and target names from source code to database schema. But you may want to customize the behavior to avoid collision or for some other reasons. There's majorly two kinds of attribute to do the mapping:

• Attributes from System.ComponentModel.DataAnnotations: common attributes for data validation.
• Attributes from System.ComponentModel.DataAnnotations.Schema: attributes for schema mapping.

Type mapping

Mapping a field

To map type of a filed in a table,

using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

public class Movie
{
    public int Id { get; set; }
    [Column(TypeName = "varchar")]
    public string? Title { get; set; }
    public DateTime ReleaseDate { get; set; }
    public string? Synopsis { get; set; }
}

Naming mapping

using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

[Table("Film")]
public class Movie
{
    public int Id { get; set; }
    public string? Title { get; set; }
    public DateTime ReleaseDate { get; set; }
    [Column("Abstract", TypeName = "varchar(128)", Order = 3)]
    public string? Synopsis { get; set; }
}

TIP

It's kind of weird that TableAttribute should be appiled on a data model instead of a DbSet

Constraint mapping

using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

public class Movie
{
    [Key] // primary key
    public int Unique { get; set; }
    [Required] // marks not null
    [MaxLength(128)] // max length in database
    public string? Title { get; set; }
    public DateTime ReleaseDate { get; set; }
    public string? Synopsis { get; set; }
}

Fluent mapping

Entity framework core also has a way to register mapping with fluent builder.

public class MoviesContext : DbContext
{
    public DbSet<Movie> Movies => Set<Movie>();
    protected override void OnModelCreating(ModelBuilder modelBuilder) // [!code highlight]
    {
        modelBuilder.Entity<Movie>() // which entity model to map
            .ToTable("Film")
            .HasKey(x => x.Id);
        modelBuilder.Entity<Movie>()
            .Property(x => x.Title) // pick a field from model
            .HasColumnType("varchar")
            .HasMaxLength(128)
            .IsRequired();
    }
}

Better organized mapping

If you got a lot entity model to map, OnModelCreating can be a complete mess. To separate those registrations, create different classes to handle that.

// some other files
class MovieMapper : IEntityTypeConfiguration<Movie>
{
    public void Configure(EntityTypeBuilder<Movie> builder)
    {
        // no more entity model selection since `builder` is a `EntityTypeBuilder<Movie>`
        builder.ToTable("Film").HasKey(x => x.Id);
        builder.Property(x => x.Title)
        .HasColumnType("varchar")
        .HasMaxLength(128)
        .IsRequired();
    }
}

// MovieContext.cs
public class MoviesContext : DbContext
{
    public DbSet<Movie> Movies => Set<Movie>();
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.ApplyConfiguration(new MovieMapper()); // [!code highlight]
    }
}

INFO

Unfortunately, C# doesn't allow static classes to implement interface, it could be cleaner if just pass a delegate from static class.