Unique Key constraints for multiple columns in Entity Framework

Question

With Entity Framework 6.1, you can now do this:

[Index("IX_FirstAndSecond", 1, IsUnique = true)]
public int FirstColumn { get; set; }

[Index("IX_FirstAndSecond", 2, IsUnique = true)]
public int SecondColumn { get; set; }

The second parameter in the attribute is where you can specify the order of the columns in the index.
More information: MSDN

I found three ways to solve the problem.

Unique indexes in EntityFramework Core:

First approach:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
   modelBuilder.Entity<Entity>()
   .HasIndex(p => new {p.FirstColumn , p.SecondColumn}).IsUnique();
}

The second approach to create Unique Constraints with EF Core by using Alternate Keys.

Examples

One column:

modelBuilder.Entity<Blog>().HasAlternateKey(c => c.SecondColumn).HasName("IX_SingeColumn");

Multiple columns:

modelBuilder.Entity<Entity>().HasAlternateKey(c => new [] {c.FirstColumn, c.SecondColumn}).HasName("IX_MultipleColumns");

EF 6 and below:

---

First approach:

dbContext.Database.ExecuteSqlCommand(string.Format(
                        @"CREATE UNIQUE INDEX LX_{0} ON {0} ({1})", 
                                 "Entitys", "FirstColumn, SecondColumn"));

This approach is very fast and useful but the main problem is that Entity Framework doesn't know anything about those changes!

---

Second approach:
I found it in this post but I did not tried by myself.

CreateIndex("Entitys", new string[2] { "FirstColumn", "SecondColumn" },
              true, "IX_Entitys");

The problem of this approach is the following: It needs DbMigration so what do you do if you don't have it?

---

Third approach:
I think this is the best one but it requires some time to do it. I will just show you the idea behind it: In this link http://code.msdn.microsoft.com/CSASPNETUniqueConstraintInE-d357224a you can find the code for unique key data annotation:

[UniqueKey] // Unique Key 
public int FirstColumn  { get; set;}
[UniqueKey] // Unique Key 
public int SecondColumn  { get; set;}

// The problem hier
1, 1  = OK 
1 ,2  = NO OK 1 IS UNIQUE

The problem for this approach; How can I combine them? I have an idea to extend this Microsoft implementation for example:

[UniqueKey, 1] // Unique Key 
public int FirstColumn  { get; set;}
[UniqueKey ,1] // Unique Key 
public int SecondColumn  { get; set;}

Later in the IDatabaseInitializer as described in the Microsoft example you can combine the keys according to the given integer. One thing has to be noted though: If the unique property is of type string then you have to set the MaxLength.

If you're using Code-First, you can implement a custom extension HasUniqueIndexAnnotation

using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Entity.Infrastructure.Annotations;
using System.Data.Entity.ModelConfiguration.Configuration;

internal static class TypeConfigurationExtensions
{
    public static PrimitivePropertyConfiguration HasUniqueIndexAnnotation(
        this PrimitivePropertyConfiguration property, 
        string indexName,
        int columnOrder)
    {
        var indexAttribute = new IndexAttribute(indexName, columnOrder) { IsUnique = true };
        var indexAnnotation = new IndexAnnotation(indexAttribute);

        return property.HasColumnAnnotation(IndexAnnotation.AnnotationName, indexAnnotation);
    }
}

Then use it like so:

this.Property(t => t.Email)
    .HasColumnName("Email")
    .HasMaxLength(250)
    .IsRequired()
    .HasUniqueIndexAnnotation("UQ_User_EmailPerApplication", 0);

this.Property(t => t.ApplicationId)
    .HasColumnName("ApplicationId")
    .HasUniqueIndexAnnotation("UQ_User_EmailPerApplication", 1);

Which will result in this migration:

public override void Up()
{
    CreateIndex("dbo.User", new[] { "Email", "ApplicationId" }, unique: true, name: "UQ_User_EmailPerApplication");
}

public override void Down()
{
    DropIndex("dbo.User", "UQ_User_EmailPerApplication");
}

And eventually end up in database as:

CREATE UNIQUE NONCLUSTERED INDEX [UQ_User_EmailPerApplication] ON [dbo].[User]
(
    [Email] ASC,
    [ApplicationId] ASC
)

The answer from niaher stating that to use the fluent API you need a custom extension may have been correct at the time of writing. You can now (EF core 2.1) use the fluent API as follows:

modelBuilder.Entity<ClassName>()
            .HasIndex(a => new { a.Column1, a.Column2}).IsUnique();

You need to define a composite key.

With data annotations it looks like this:

public class Entity
 {
   public string EntityId { get; set;}
   [Key]
   [Column(Order=0)]
   public int FirstColumn  { get; set;}
   [Key]
   [Column(Order=1)]
   public int SecondColumn  { get; set;}
 }

You can also do this with modelBuilder when overriding OnModelCreating by specifying:

modelBuilder.Entity<Entity>().HasKey(x => new { x.FirstColumn, x.SecondColumn });

Completing @chuck answer for using composite indices with foreign keys.

You need to define a property that will hold the value of the foreign key. You can then use this property inside the index definition.

For example, we have company with employees and only we have a unique constraint on (name, company) for any employee:

class Company
{
    public Guid Id { get; set; }
}

class Employee
{
    public Guid Id { get; set; }
    [Required]
    public String Name { get; set; }
    public Company Company  { get; set; }
    [Required]
    public Guid CompanyId { get; set; }
}

Now the mapping of the Employee class:

class EmployeeMap : EntityTypeConfiguration<Employee>
{
    public EmployeeMap ()
    {
        ToTable("Employee");

        Property(p => p.Id)
            .HasDatabaseGeneratedOption(DatabaseGeneratedOption.None);

        Property(p => p.Name)
            .HasUniqueIndexAnnotation("UK_Employee_Name_Company", 0);
        Property(p => p.CompanyId )
            .HasUniqueIndexAnnotation("UK_Employee_Name_Company", 1);
        HasRequired(p => p.Company)
            .WithMany()
            .HasForeignKey(p => p.CompanyId)
            .WillCascadeOnDelete(false);
    }
}

Note that I also used @niaher extension for unique index annotation.

For those finding this looking for a 2021 solution, the working version of the accepted answer should now look like this:

[Index(nameof(FirstColumn), nameof(SecondColumn), IsUnique = true)]
public class Entity
 {
   [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
   public string EntityId { get; set;}
   public int FirstColumn  { get; set;}
   public int SecondColumn  { get; set;}
 }

So that the annotation should live on the model and not the individual columns. Also note the nameof() syntax.

This answer was derived from the official documentation: https://docs.microsoft.com/en-us/ef/core/modeling/indexes?tabs=data-annotations