NHibernate select top N with condition on children using fetch

Question

I have simple User entity:

public class User
{
    public virtual int Id { get; set; }
    public virtual DateTime CreationDate { get; set; }
    public virtual DateTime ModifiedDate { get; set; }

    public virtual string Email { get; set; }
    public virtual string Name { get; set; }

    public virtual IList<Phone> Phones { get; set; }
}

public class Phone
{
    public virtual string CountryCode { get; set; }
    public virtual string Code { get; set; }
    public virtual string Number { get; set; }
    public virtual string Comment { get; set; }
}

My mappings defined as this:

public class UserMap : ClassMap<User>
{
    public UserMap ()
    {
        this.Table ("Users");

        this.Id (x => x.Id).CustomSqlType ("bigint").GeneratedBy.HiLo ("1000");
        this.Map (x => x.CreationDate);
        this.Map (x => x.ModifiedDate).Column ("LastUpdatedDate");
        this.Map (x => x.Email).Length (255).Not.Nullable ().Unique ();
        this.Map (x => x.Name).Column ("UserName").Length (255);

        this.HasMany (x => x.Phones).Inverse ();
    }
}

public class PhoneMap : ClassMap<Phone>
{
    public PhoneMap ()
    {
        this.Table ("Phones");

        this.Id ().GeneratedBy.Identity ();
        this.Map (x => x.CountryCode).Length (5);
        this.Map (x => x.Code).Length (10);
        this.Map (x => x.Number).Length (50).Not.Nullable ();
        this.Map (x => x.Comment).Length (255);
    }
}

Additional conventions here:

PrimaryKey.Name.Is (x => "Id"),
ForeignKey.EndsWith ("Id"),
DefaultAccess.Property (),
DefaultCascade.All ()

I need to select top 100 users with Phones and whose name starts with "A". But I need to load user objects with Phones in them.

So I do this query:

var users =
(
    from user in session.Query<User> ()
    where
        user.Name.StartsWith ("a")
        &&
        user.Phones.Any ()
    select user
)
    .Fetch (x => x.Phones)
    .Take (100)
    .ToArray ();

And I only got 72 users.

Why? Well, because NHibernate generates single TOP N select with left outer join and SQL returns several records for the same user entity because some users do have more that one phone. But it's all counts against TOP N - so I get 100 records of users joined with phones, but only 72 of them are unique entities.

Is there a proper way to do it?

Answer 1

You have to split queries to subselects. Where inner subselect should do pagination and outer should do fetching:

var top100users =
(
    from user in session.Query<User>()
    where user.Name.StartsWith("a") &&
          user.Phones.Any()
    select user
)
.Take(100);

var users =
(
    from user in session.Query<User>()
    where top100users.Contains(user)
    select user
)
.Fetch (x => x.Phones)
.ToArray();

And this will generate single sql query which will behave as you expect.

Answer 2

Well, the only possible workaround I came up with is to firstly remove Fetch from query so it became like this:

var users =
    (
        from user in session.Query<User> ()
        where
            user.Name.StartsWith (prefix)
            &&
            user.Phones.Any ()
        select user
    )
    .Take (100)
    .ToList ();

Then after that code add something like this that force to load at least one entity:

users.ForEach (x => x.Phones.Any ());

And in mappings set batch size to 100 (or at least 50):

public class UserMap : ClassMap<User>
{
    public UserMap ()
    {
        this.Table ("Users");

        this.Id (x => x.Id).CustomSqlType ("bigint").GeneratedBy.HiLo ("1000");
        this.Map (x => x.CreationDate);
        this.Map (x => x.ModifiedDate).Column ("LastUpdatedDate");
        this.Map (x => x.Email).Length (255).Not.Nullable ().Unique ();
        this.Map (x => x.Name).Column ("UserName").Length (255);

        this.HasMany (x => x.Phones).Inverse ().BatchSize (50);
    }
}

Or via conventions (tho it can be not so graceful for some systems):

PrimaryKey.Name.Is (x => "Id"),
ForeignKey.EndsWith ("Id"),
DefaultAccess.Property (),
DefaultCascade.All (),
DynamicUpdate.AlwaysTrue (),
new CollectionConventionBuilder ().Always (x => x.BatchSize (50))

Btw, in pure SQL the task could be solved rather simple with "for xml":

select top 100
    u.Id,
    u.CreationDate,
    u.LastUpdatedDate,
    u.Email,
    u.UserName,
    (
        select
            p.CountryCode,
            p.Code,
            p.Number,
            p.Comment
        from
            dbo.Phones as p
        where
            p.UserId = u.Id
        for xml path ('Phone'), root ('Phones'), type
    ) as '*'
from
    dbo.Users as u
where
    u.UserName like @0
    and
    exists (select top 1 p.Id from dbo.Phones as p where p.UserId = u.Id)
for xml path ('User'), root ('Root'), type

I wish NHibernate could load aggregate roots from "for xml" queries when ordered to.

Answer 3

You need to use a subquery (for the paging) and a transformer to get distinct users, I'm not sure if this is possible in the NHibernate Linq provider, so doing it using QueryOver:

var sub_query = QueryOver.Of<User>() 
    .Where (Restrictions.On<User>(x => x.Name).IsLike("a%")) 
    .JoinQueryOver(x => x.Phones, JoinType.InnerJoin) 
    .Take (100)
    .Select(x => x.Id);

var users = session.QueryOver<User>() 
    .WithSubquery.WhereProperty (x => x.Id).In (sub_query) 
    .Fetch (x => x.Phones).Eager
    .TransformUsing (Transformers.DistinctRootEntity) 
    .List ();