Creating and configuring a model

• 10/13/2020
• 2 minutes to read
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Entity Framework uses a set of conventions to build a model based on the shape of your entity classes. You can specify additional configuration to supplement and/or override what was discovered by convention.

This article covers configuration that can be applied to a model targeting any data store and that which can be applied when targeting any relational database. Providers may also enable configuration that is specific to a particular data store. For documentation on provider specific configuration see the Database Providers section.

Use fluent API to configure a model

You can override the OnModelCreating method in your derived context and use the ModelBuilder API to configure your model. This is the most powerful method of configuration and allows configuration to be specified without modifying your entity classes. Fluent API configuration has the highest precedence and will override conventions and data annotations.

using Microsoft.EntityFrameworkCore;

namespace EFModeling.FluentAPI.Required
{
    internal class MyContext : DbContext
    {
        public DbSet<Blog> Blogs { get; set; }

        #region Required
        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Blog>()
                .Property(b => b.Url)
                .IsRequired();
        }
        #endregion
    }

    public class Blog
    {
        public int BlogId { get; set; }
        public string Url { get; set; }
    }
}

Grouping configuration

To reduce the size of the OnModelCreating method all configuration for an entity type can be extracted to a separate class implementing IEntityTypeConfiguration<TEntity>.

public class BlogEntityTypeConfiguration : IEntityTypeConfiguration<Blog>
{
    public void Configure(EntityTypeBuilder<Blog> builder)
    {
        builder
            .Property(b => b.Url)
            .IsRequired();
    }
}

Then just invoke the Configure method from OnModelCreating.

new BlogEntityTypeConfiguration().Configure(modelBuilder.Entity<Blog>());

It is possible to apply all configuration specified in types implementing IEntityTypeConfiguration in a given assembly.

modelBuilder.ApplyConfigurationsFromAssembly(typeof(BlogEntityTypeConfiguration).Assembly);

Use data annotations to configure a model

You can also apply attributes (known as Data Annotations) to your classes and properties. Data annotations will override conventions, but will be overridden by Fluent API configuration.

using System.ComponentModel.DataAnnotations;
using Microsoft.EntityFrameworkCore;

namespace EFModeling.DataAnnotations.Required
{
    internal class MyContext : DbContext
    {
        public DbSet<Blog> Blogs { get; set; }
    }

    #region Required
    public class Blog
    {
        public int BlogId { get; set; }

        [Required]
        public string Url { get; set; }
    }
    #endregion
}