static IFreeSql fsql = new FreeSql.FreeSqlBuilder()
  .UseConnectionString(FreeSql.DataType.Sqlite, @"Data Source=document.db")
  .UseAutoSyncStructure(true) //automatically synchronize the entity structure to the database
  .Build(); //be sure to define as singleton mode

class Song {
  [Column(IsIdentity = true)]
  public int Id { get; set; }
  public string Title { get; set; }
  public string Url { get; set; }
  public DateTime CreateTime { get; set; }
  
  public ICollection<Tag> Tags { get; set; }
}
class Song_tag {
  public int Song_id { get; set; }
  public Song Song { get; set; }
  
  public int Tag_id { get; set; }
  public Tag Tag { get; set; }
}
class Tag {
  [Column(IsIdentity = true)]
  public int Id { get; set; }
  public string Name { get; set; }
  
  public int? Parent_id { get; set; }
  public Tag Parent { get; set; }
  
  public ICollection<Song> Songs { get; set; }
  public ICollection<Tag> Tags { get; set; }
}

//OneToOne、ManyToOne
fsql.Select<Tag>().Where(a => a.Parent.Parent.Name == "English").ToList();

//OneToMany
fsql.Select<Tag>().IncludeMany(a => a.Tags, then => then.Where(sub => sub.Name == "foo")).ToList();

//ManyToMany
fsql.Select<Song>()
  .IncludeMany(a => a.Tags, then => then.Where(sub => sub.Name == "foo"))
  .Where(s => s.Tags.Any(t => t.Name == "Chinese"))
  .ToList();

//Other
fsql.Select<YourType>()
  .Where(a => a.IsDelete == 0)
  .WhereIf(keyword != null, a => a.UserName.Contains(keyword))
  .WhereIf(role_id > 0, a => a.RoleId == role_id)
  .Where(a => a.Nodes.Any(t => t.Parent.Id == t.UserId))
  .Count(out var total)
  .Page(page, size)
  .OrderByDescending(a => a.Id)
  .ToList()

fsql.Select<Song>().Where(a => new[] { 1, 2, 3 }.Contains(a.Id)).ToList();

fsql.Select<Song>().Where(a => a.CreateTime.Date == DateTime.Today).ToList();

fsql.Select<Song>().OrderBy(a => Guid.NewGuid()).Limit(10).ToList();

fsql.Select<Song>().ToList(a => new
{
    a.Id,
    Tags = fsql.Select<Tag>().ToList(),
    SongTags = fsql.Select<SongTag>().Where(b => b.TopicId == a.Id).ToList()
});

[Transactional]
public void Add() {
  var repo = ioc.GetService<BaseRepository<Tag>>();
  repo.DbContextOptions.EnableCascadeSave = true;

  var item = new Tag {
    Name = "testaddsublist",
    Tags = new[] {
      new Tag { Name = "sub1" },
      new Tag { Name = "sub2" }
    }
  };
  repo.Insert(item);
}