How to select from a subselect on NHibernate

How can I map this SQL using NHibernate Criteria API?开发者_如何学编程

Sql:

SELECT COUNT(*) FROM (
    SELECT FirstName, LastName FROM Employees GROUP BY FirstName, LastName
) AS Query

This is a very very simple query, my query has a SubSelect much more complex.

So, any idea?

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I found a solution for the question, it's a very big hack but it works as expected.

I had to get the generated SQL and surround it with the SELECT COUNT(*) query. Here is the code to do that:

public ISQLQuery BuildCountQuery(ICriteria criteria)
{
    CriteriaImpl c = (CriteriaImpl)criteria;
    SessionImpl s = (SessionImpl)c.Session;
    string entityOrClassName = ExtractRealClassName(c);

    SessionFactoryImpl factory = (SessionFactoryImpl)s.SessionFactory;
    String[] implementors = factory.GetImplementors(entityOrClassName);
    string implementor = implementors.Length == 0 ? null : implementors[0];
    var persister = (IOuterJoinLoadable)factory.GetEntityPersister(implementor);
    CriteriaLoader loader = new CriteriaLoader(persister, factory, c, implementor, s.EnabledFilters);
    SqlString sql = loader.SqlString.Insert(0, "SELECT COUNT(*) FROM (");
    sql = sql.Append(") AS Query");

    var parameters = loader.Translator.CollectedParameters;
    var sqlQuery = this.session.CreateSQLQuery(sql.ToString());
    for (int i = 0; i < parameters.Count; i++)
        sqlQuery.SetParameter(i, parameters.ElementAt(i).Value, parameters.ElementAt(i).Type);

    return sqlQuery;
}

private string ExtractRealClassName(CriteriaImpl criteria)
{
    Type rootEntityType = criteria.GetRootEntityTypeIfAvailable();
    if (rootEntityType.GetInterfaces().Contains(typeof(INHibernateProxy)))
        return criteria.GetRootEntityTypeIfAvailable().BaseType.FullName;
    else
        return criteria.EntityOrClassName;
}

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DetachedCriteria criteriaEmployees = DetachedCriteria.For<Employees>();
criteriaEmployees.SetProjection(Projections.CountDistinct("FirstName"));
ICriteria executableCriteria = criteriaEmployees.GetExecutableCriteria(Session);
int count = executableCriteria.UniqueResult<int>();

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DetachedCriteria can be used to create subqueries. Some examples are in the documentation.