CommandValidator.cs

public class CommandValidator : AbstractValidator<Command>
{
    private ApplicationDbContext _dbContext;

    public CommandValidator(ApplicationDbContext dbContext)
    {
        _dbContext = dbContext;

        RuleFor(c => c.Body).NotEmpty();

        RuleFor(c => c.Slug).NotEmpty()
            .Matches("^[a-z0-9]+(?:-[a-z0-9]+)*$")
            .WithMessage("Please enter a valid slug, using only alphanumeric characters and hyphens e.g. test-post")
            .Must(BeUniqueSlug).WithMessage("Slug already in use, try a different one");

        RuleFor(c => c.Title).NotEmpty();                
    }

    private bool BeUniqueSlug(string slug)
    {
        return _dbContext.Posts.Count(x => x.Slug == slug) == 0;
    }
}