Loading MetaData from Partial Class in Aspnet Core

Hello everyone, I'm building a project in MVC template using AspNet Core.

I've used Entity Framework and I've scaffolded an existing DB. Now, I want to add some Data Annotations to some classes, but I don't wanna edit the class autogenerated by the scaffolding, so I've tried with the Metadata and the overriding of an existing method, the saveChanges().

Users.cs autogenerated by Scaffolding
public partial class Users
{
    public int UserId { get; set; }
    ...
    // If I have [MaxLength(5, ErrorMessage = "Too short")] here, it works
    public string Email { get; set; }
}

UsersMetadata.cs (also tried Users.Metadata.cs or else, nothing changed)
[ModelMetadataType(typeof(UsersModelMetaData))]
public partial class Users { }

public class UsersModelMetaData
{
    [MaxLength(5, ErrorMessage = "Too short")]
    public string Email { get; set; }
}

MyContext : DbContext Class
public override int SaveChanges()
{
    var entities = from e in ChangeTracker.Entries()
        where e.State == EntityState.Added
        || e.State == EntityState.Modified
        select e.Entity;
    foreach (var entity in entities)
    {
        var validationContext = new ValidationContext(entity);
        Validator.ValidateObject(entity, validationContext, validateAllProperties: true);
    }

    return base.SaveChanges();
}

So, even if this seems to be the correct solution (i've search all morning), it doesn't work: the problem seems to be that the Data Annotation inside UsersModelMetaData aren't read, because if I put the Data Annotation directly inUsers.cs file, method saveChanges() will throw an exception.
Notice that I had to override the saveChanges() method because, as I understood, in AspNet Core there is no DbEntityValidationException, so in this way I can detectValidationException.

I did found this solution -> https://stackoverflow.com/a/30127682/6070423 <- but it's based on AspNet, and using AspNet Core I cannot useAssociatedMetadataTypeTypeDescriptionProvider.

Any idea how I could resolve this?