Efcore: Create a 'IsUnique' attribute

[BrunoBlanes]

I would like to not need to type all of this just to set a few columns as unique in the database. This is just for one case, I have an entire set of 20 lines just to set unique constrains in my OnModelCreating method.
```c#
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity(company =>
{
company.HasIndex(x => x.IE).IsUnique();
company.HasIndex(x => x.IM).IsUnique();
company.HasIndex(x => x.CNPJ).IsUnique();
});
}

<!-- Describe the solution you'd like -->
Instead, why not create an `[Unique]` attribute to be used like this:

```c#
public class Company : Contact
{
    [Unique]
    [Required]
    [DataMember]
    public string CNPJ { get; set; }

    [Unique]
    [DataMember]
    public string? IE { get; set; }

    [Unique]
    [DataMember]
    public string? IM { get; set; }

    public Company() : base()
    {
        CNPJ = string.Empty;
    }
}

I don't believe this exists, given that I couldn't find any documentation that proved otherwise and also couldn't find any open issues on the matter. Please consider.

[smitpatel]

Checkout IndexAttribute which allows you to configure indexes and market them as unique. This feature requires using EF Core 5.0. Current pre-release version on nuget, 5.0-rc2 contains it.

[BrunoBlanes]

Thanks @smitpatel that does the job, but isn't it possible to have a simpler attribute per property? Or was there a design decision that said otherwise?

[ajcvickers]

@BrunoBlanes Is it your intention to create a unique constraint or a unique index in the database, or does it not matter to you which of these is created?

[BrunoBlanes]

You got me there, they seem to me to do the same thing, but if I studied properly what I want is a unique constraint, which is to have a single value per cell on a specific column.

[ajcvickers]

@BrunoBlanes Thanks. So, to ask my question in a slightly different way: the reason you don't want to use [Index] is not because you care about the (largely academic) difference between a constraint and an index? (I believe most databases will create a unique index to implement the unique constraint anyway.)

[BrunoBlanes]

It is not.
And it is no that I don't want to use it either, in fact I am using it now that you've showed me the attribute, but I think it is easier to understand which properties a specific column has on the database just by looking at its property on the model class and seeing the attributes on top of it.
In other words, if you have a model with 10 properties you want to set a unique constraint to, your [Index] attribute would be about two lines long on top of a class. I think it makes more sense to add it on a per property basis, and not globally.

[BrunoBlanes]

c# [Index(nameof(IE), nameof(IM), nameof(CNPJ), IsUnique = true)]
This is what I had to do in one case for 3 properties, if I could have set one [Unique] attribute on top of each one of those properties it would make more sense for what I need.

[smitpatel]

On side note,

[Index(nameof(IE), nameof(IM), nameof(CNPJ), IsUnique = true)]

Creates a unique index over all 3 properties (composite index). In order to define own separate unique index over each property following should be used

[Index(nameof(IE), IsUnique = true)]
[Index(nameof(IM), IsUnique = true)]
[Index(nameof(CNPL), IsUnique = true)]

[BrunoBlanes]

Hum, that makes sense, I didn't read enough through the docs to see that, thanks @smitpatel.

[ajcvickers]

We discussed this some more and created two new issues to cover points raised here: #23153 and #23154.

[BrunoBlanes]

@ajcvickers thanks for considering.