How to count all posts belonging to multiple tags in NHibernate? - sql

Question

I have a many to many relationship:
A post can have many tags
A tag can have many posts
Models:
public class Post
{
public virtual string Title { get; set; }
public virtual string Content{ get; set; }
public virtual User User { get; set; }
public virtual ICollection<Tag> Tags { get; set; }
}
public class Tag
{
public virtual string Title { get; set; }
public virtual string Description { get; set; }
public virtual User User { get; set; }
public virtual ICollection<Post> Posts { get; set; }
}
I want to count all posts that belong to multiple tags but I don't know how to do this in NHibernate. I am not sure if this is the best way to do this but I used this query in MS SQL:
SELECT COUNT(*)
FROM
(
SELECT Posts.Id FROM Posts
INNER JOIN Users ON Posts.UserId=Users.Id
LEFT JOIN TagsPosts ON Posts.Id=TagsPosts.PostId
LEFT JOIN Tags ON TagsPosts.TagId=Tags.Id
WHERE Users.Username='mr.nuub' AND (Tags.Title in ('c#', 'asp.net-mvc'))
GROUP BY Posts.Id
HAVING COUNT(Posts.Id)=2
)t
But NHibernate does not allow subqueries in the from clause. It would be great if someone could show me how to do this in HQL.

Answer 1

I found a way of how to get this result without a sub query and this works with nHibernate Linq. It was actually not that easy because of the subset of linq expressions which are supported by nHibernate... but anyways
query:
var searchTags = new[] { "C#", "C++" };
var result = session.Query<Post>()
.Select(p => new {
Id = p.Id,
Count = p.Tags.Where(t => searchTags.Contains(t.Title)).Count()
})
.Where(s => s.Count >= 2)
.Count();
It produces the following sql statment:
select cast(count(*) as INT) as col_0_0_
from Posts post0_
where (
select cast(count(*) as INT)
from PostsToTags tags1_, Tags tag2_
where post0_.Id=tags1_.Post_id
and tags1_.Tag_id=tag2_.Id
and (tag2_.Title='C#' or tag2_.Title='C++'))>=2
you should be able to build your user restriction into this, I hope.
The following is my test setup and random data which got generated
public class Post
{
public Post()
{
Tags = new List<Tag>();
}
public virtual void AddTag(Tag tag)
{
this.Tags.Add(tag);
tag.Posts.Add(this);
}
public virtual string Title { get; set; }
public virtual string Content { get; set; }
public virtual ICollection<Tag> Tags { get; set; }
public virtual int Id { get; set; }
}
public class PostMap : ClassMap<Post>
{
public PostMap()
{
Table("Posts");
Id(p => p.Id).GeneratedBy.Native();
Map(p => p.Content);
Map(p => p.Title);
HasManyToMany<Tag>(map => map.Tags).Cascade.All();
}
}
public class Tag
{
public Tag()
{
Posts = new List<Post>();
}
public virtual string Title { get; set; }
public virtual string Description { get; set; }
public virtual ICollection<Post> Posts { get; set; }
public virtual int Id { get; set; }
}
public class TagMap : ClassMap<Tag>
{
public TagMap()
{
Table("Tags");
Id(p => p.Id).GeneratedBy.Native();
Map(p => p.Description);
Map(p => p.Title);
HasManyToMany<Post>(map => map.Posts).LazyLoad().Inverse();
}
}
test run:
var sessionFactory = Fluently.Configure()
.Database(FluentNHibernate.Cfg.Db.MsSqlConfiguration.MsSql2012
.ConnectionString(#"Server=.\SQLExpress;Database=TestDB;Trusted_Connection=True;")
.ShowSql)
.Mappings(m => m.FluentMappings
.AddFromAssemblyOf<PostMap>())
.ExposeConfiguration(cfg => new SchemaUpdate(cfg).Execute(false, true))
.BuildSessionFactory();
using (var session = sessionFactory.OpenSession())
{
var t1 = new Tag() { Title = "C#", Description = "C#" };
session.Save(t1);
var t2 = new Tag() { Title = "C++", Description = "C/C++" };
session.Save(t2);
var t3 = new Tag() { Title = ".Net", Description = "Net" };
session.Save(t3);
var t4 = new Tag() { Title = "Java", Description = "Java" };
session.Save(t4);
var t5 = new Tag() { Title = "lol", Description = "lol" };
session.Save(t5);
var t6 = new Tag() { Title = "rofl", Description = "rofl" };
session.Save(t6);
var tags = session.Query<Tag>().ToList();
var r = new Random();
for (int i = 0; i < 1000; i++)
{
var post = new Post()
{
Title = "Title" + i,
Content = "Something awesome" + i,
};
var manyTags = r.Next(1, 3);
while (post.Tags.Count() < manyTags)
{
var index = r.Next(0, 6);
if (!post.Tags.Contains(tags[index]))
{
post.AddTag(tags[index]);
}
}
session.Save(post);
}
session.Flush();
/* query test */
var searchTags = new[] { "C#", "C++" };
var result = session.Query<Post>()
.Select(p => new {
Id = p.Id,
Count = p.Tags.Where(t => searchTags.Contains(t.Title)).Count()
})
.Where(s => s.Count >= 2)
.Count();
var resultOriginal = session.CreateQuery(#"
SELECT COUNT(*)
FROM
(
SELECT count(Posts.Id)P FROM Posts
LEFT JOIN PostsToTags ON Posts.Id=PostsToTags.Post_id
LEFT JOIN Tags ON PostsToTags.Tag_id=Tags.Id
WHERE Tags.Title in ('c#', 'C++')
GROUP BY Posts.Id
HAVING COUNT(Posts.Id)>=2
)t
").List()[0];
var isEqual = result == (int)resultOriginal;
}
As you can see at the end I do test against your original query (without the users) and it is actually the same count.

Answer 2

In HQL:
var hql = "select count(p) from Post p where p in " +
"(select t.Post from Tag t group by t.Post having count(t.Post) > 1)";
var result = session.Query(hql).UniqueResult<long>();
You can add additional criteria to the subquery if you need to specify tags or other criteria.

Answer 3

Edit : In the future I should read the questions until the last words. I would have seen in HQL...
After some seach, realizing that RowCount removes any grouping in the query ( https://stackoverflow.com/a/8034921/1236044 ). I found a solution using QueryOver and SubQuery which I post here as information.
I find this solution interesting as it offers some modularity, and seprates the counting from the subquery itself, which can be reused as it is.
var searchTags = new[] { "tag1", "tag3" };
var userNames = new[] { "mr.nuub" };
Tag tagAlias = null;
Post postAlias = null;
User userAlias = null;
var postsSubquery =
QueryOver.Of<Post>(() => postAlias)
.JoinAlias(() => postAlias.Tags, () => tagAlias)
.JoinAlias(() => postAlias.User, () => userAlias)
.WhereRestrictionOn(() => tagAlias.Title).IsIn(searchTags)
.AndRestrictionOn(() => userAlias.UserName).IsIn(userNames)
.Where(Restrictions.Gt(Projections.Count<Post>(p => tagAlias.Title), 1));
var numberOfPosts = session.QueryOver<Post>()
.WithSubquery.WhereProperty(p => p.Id).In(postsSubquery.Select(Projections.Group<Post>(p => p.Id)))
.RowCount();
Hope this will help