nhibernate queryover join with subquery to get aggregate column

Question

I have been searching for several hours now how to do this, but can't seem to find anything to help me.

Here is the database model:

This is the SQL query I am trying to run:

SELECT b.*, a.Assignments FROM Branch b LEFT JOIN (
        SELECT b.BranchID , COUNT(ab.BranchID) AS Assignments
        FROM Branch b LEFT JOIN AssignmentBranch ab ON b.BranchID = ab.BranchID
        GROUP BY b.BranchID
      ) a ON b.BranchID = a.BranchID

So, basically, I want to return a list of branches and a new column that represents the number of assignments for that branch.

Branch model

public class Branch : IEntity<int>
{
    public virtual int ID
    {
        get;
        set;
    }

    public virtual string Name { get; set; }

    public virtual IList<AssignmentBranch> Assignments { get; set; }

}

AssignmentBranch model

public class AssignmentBranch : IEntity<int>
{
    public virtual int ID
    {
        get;
        set;
    }

    public virtual DateTime AssignedOn { get; set; }

    public virtual Branch Branch { get; set; }
}

Here is my NHibernate configuration:

<class name="Branch" table="Branch">

<id name="ID" column="BranchID">
  <generator class="identity"></generator>
</id>

<property name="Name"/>

<bag name="Assignments" cascade="none" inverse="true">
  <key column="BranchID"/>
  <one-to-many class="AssignmentBranch"/>
</bag>

 <class name="AssignmentBranch" table="AssignmentBranch">

<id name="ID" column="AssignmentBranchID">
  <generator class="identity"></generator>
</id>

<property name="AssignedOn" />
<property name="FromDate" />
<property name="ToDate" />

<many-to-one name="Assignment" column="AssignmentID" />
<many-to-one name="Branch" column="BranchID" />

I have tried this a number of ways, but I can't seem to find a way to join with a sub-query using QueryOver.

I tried like this:

 // aliases
 Branch branch = null; AssignmentBranch assignment = null;

 var subquery = QueryOver.Of<Branch>(() => branch)
     .Where(() => branch.Project.ID == projectID)
     .JoinQueryOver<AssignmentBranch>(() => branch.Assignments, ()=> assignment, 
                                   NHibernate.SqlCommand.JoinType.LeftOuterJoin)
     .SelectList(list => list
                        .SelectGroup(x=>x.ID)
                        .SelectCount(()=>assignment.ID)
                    );

     var query = session.QueryOver<Branch>(()=>branch)
                  .JoinAlias(???) // how can I join with a sub-query?
                  .TransformUsing(Transformers.AliasToBean<BranchAssignments>())
                  .List<BranchAssignments>();

Can anyone help me please? It doesn't have to be with a sub-join exactly, maybe there is another better solution out there that I am missing...

Thank you, Cosmin

Answer 1

After reading hundreds of similar questions in here, I have found the answer: a correlated sub-query. Like this:

// aliases
Branch branch = null; AssignmentBranch assignment = null;

var subquery = QueryOver.Of<AssignmentBranch>(() => assignment)
    .Where(() => assignment.Branch.ID == branch.ID)
    .ToRowCountQuery();

var query = session.QueryOver<Branch>(() => branch)
     .Where(() => branch.Project.ID == projectID)
     .SelectList
     (
         list => list
         .Select(b => b.ID)
         .Select(b => b.Name)
         .SelectSubQuery(subquery)
     )
     .TransformUsing(Transformers.AliasToBean<BranchAssignments>())
     .List<BranchAssignments>();

The similar question I got my answer from is this one.

Answer 2

its not that easy with QueryOver, because it is currently not possible to have statements in the FROM clause. One thing that comes to my mind (not the most efficient way i think)

var branches = session.QueryOver<Branch>().Future();

var assignmentMap = session.QueryOver<BranchAssignment>()
    .Select(
        Projections.Group<BranchAssignment>(ab => ab.Branch.Id).As("UserId"),
        Projections.RowCount())
    .Future<object[]>()
    .ToDictionary(o => (int)o[0], o => (int)o[1]);

return branches.Select(b => new { Branch = branch, AssignmentCount = assignmentMap[branch.Id] });

with LINQ it would be

var branchesWithAssignementCount = session.Query<Branch>()
    .Select(b => new { Branch = b, AssignmentCount = b.Branch.Count })
    .ToList();