Question

I am trying to reproduce a query using the Nhibernate Criteria API and cannot work out how to add a criteria clause that allows me to compare two values from different tables.

The best trivial example I could come up with….

SELECT e.LastName
FROM Employee e
JOIN Chair c ON c.ChairId = e.ChairId
WHERE e.Weight > c.MaxLoad

My basic Nhibernate criteria

ICriteria criteria = base.Session.CreateCriteria(typeof(Employee));
criteria.CreateAlias("Employee.Chairid", "Chair", JoinType.InnerJoin);

One overload of create alias has an additional parameter of “withClause” which seems to be the suggested way to achieving this but for the life of me I cannot find an example of the syntax I would need to achieve this.

I think I need something along the lines of …

criteria.Add(Expression.Ge("Employee.Weight", "Chair.MaxLoad"));

but this obviously doesn’t work as the second parameter will be processed as a string value.

Any help appreciated.

Answer 1

var employeeNames = Session.CreateCriteria(typeof(Employee))
    .CreateAlias("Chair", "c") // InnerJoin is implied
    .Add(Restrictions.GtProperty("Weight", "c.MaxLoad"))
    .SetProjection(Projections.Property("LastName"))
    .List<string>();

The Restrictions.XxProperty() constraints are what you are looking for, which compare two properties against each other.