public class Regulatory
    {
        public Regulatory()
        {
            Tags = new HashSet<Tag>();
        }

        [Display(Name = "Идентификатор документа")]
        public int Id { get; set; }

        [Display(Name = "Теги")]
        public virtual ICollection<Tag> Tags { get; set; }

    }

public class Tag
    {
        public Tag()
        {
            Articles  = new HashSet<Article>();
            Events  = new HashSet<Event>();
            Regulatories  = new HashSet<Regulatory>();
        }

        [Display(Name = "Идентификатор тега")]
        public int Id { get; set; }

        [Display(Name = "Статьи")]
        public virtual ICollection<Article> Articles { get; set; }

        [Display(Name = "События")]
        public virtual ICollection<Event> Events { get; set; }

        [Display(Name = "Акты")]
        public virtual ICollection<Regulatory> Regulatories { get; set; }

    }

var regulatories = Repository
                .AllIncluding((Regulatory reg) => reg.Tags)
                .Where(r => r.Status == DocumentStatus.Published)
                .Where(p => category == null || p.Tags.Any(t => t.Name == category))
                .OrderBy(p => p.Id)
                .ToList();

var tags = Repository.Tags.Where(t => t.Regulatories != null).ToList();

var tags = repository.Tags
                .Where(r => r.Regulatories.Count > 0)
                .ToList();