Index Attribute

Entity Framework 6 provides the Index attribute to create an index on a particular column in the database.

public class Book
{
public int Id { get; set; }
[Index]
public string Title { get; set; }
public string Category { get; set; }
public int AuthorId { get; set; }
public Author Author { get; set; }
}

But in EF Core, indexes cannot be created using data annotations. You have to use the Fluent API to specify an index on a column as shown below.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Book>()
.HasIndex(b => b.Title);
}

Now in EF Core, the new Index attribute can be placed on an entity type to specify an index for one or more columns.

[Index(nameof(Title), IsUnique = true)]
public class Book
{
public int Id { get; set; }
public string Title { get; set; }
public string Category { get; set; }
public int AuthorId { get; set; }
public Author Author { get; set; }
}

Now when you add a migration, you will see that the index is created for the Title column.

CREATE TABLE [dbo].[Books] (
[Id] INT IDENTITY (1, 1) NOT NULL,
[Title] NVARCHAR (450) NULL,
[Category] NVARCHAR (MAX) NULL,
[AuthorId] INT NOT NULL,
CONSTRAINT [PK_Books] PRIMARY KEY CLUSTERED ([Id] ASC),
CONSTRAINT [FK_Books_Authors_AuthorId] FOREIGN KEY ([AuthorId]) REFERENCES [dbo].[Authors] ([Id]) ON DELETE CASCADE
);
GO
CREATE NONCLUSTERED INDEX [IX_Books_AuthorId]
ON [dbo].[Books]([AuthorId] ASC);
GO
CREATE UNIQUE NONCLUSTERED INDEX [IX_Books_Title]
ON [dbo].[Books]([Title] ASC) WHERE ([Title] IS NOT NULL);

You can also use the Index attribute to specify an index spanning multiple columns.

[Index(nameof(FirstName), nameof(LastName), IsUnique = true)]
public class Author
{
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public ICollection<Book> Books { get; set; }
}

For SQL Server, Migrations will then generate the following SQL.

CREATE TABLE [dbo].[Authors] (
[Id] INT IDENTITY (1, 1) NOT NULL,
[FirstName] NVARCHAR (450) NULL,
[LastName] NVARCHAR (450) NULL,
CONSTRAINT [PK_Authors] PRIMARY KEY CLUSTERED ([Id] ASC)
);
GO
CREATE UNIQUE NONCLUSTERED INDEX [IX_Authors_FirstName_LastName]
ON [dbo].[Authors]([FirstName] ASC, [LastName] ASC)
WHERE ([FirstName] IS NOT NULL AND [LastName] IS NOT NULL);