NHibernate：如何使用CreateSQLQuery返回标量值列表（从一列）？

Question

What is the best/cleanest method of calling a native SQL returning a list of scalar values (ints in my case) having ISession object?

I am trying to run the following but I am always getting some errors:

var query = _session.CreateSQLQuery("SELECT Id FROM SomeTable");

A. var ids = query.List<int>(); // <-- throws ArgumentNullException "Value cannot be null.\r\nParameter name: item"
B. var ids = query.List(); returns one element array with no valid information.
C. query.SetResultTransformer(NHibernate.Transform.Transformers.AliasToBean<int>());
var ids = query.List<int>(); // throws PropertyNotFoundException: "Could not find a setter for property 'Id' in class 'System.Int32'"

Is there a way to retrieve a list of ints without creating an entity class containing just one int property named Id?

Answer 1

When you call List from CreateSQLQuery you will get a instance of IList and it internally it is a List<object> . If you have null values on this result, you will not be able to convert to int because it is a value type. So, a solution is to iterate over the result and convert it when it is a valid integer. For sample:

var values = _session.CreateSQLQuery("SELECT Id FROM SomeTable").List();
var ids = new List<int>();

foreach (var item in values)
{
   if (item != null)
      ids.Add(Convert.ToInt32(item));
}

If this is a mapped table on the nhibernate scope, you could use LINQ to do this, for sample:

var ids = session.Query<SomeEntity>().Select(x => x.Id).ToList();

Answer 2

I know you are not using IQueryOver , but it is simpler, dynamic and cleaner than the way you are doing this now.

public IList<TReturn> GetValues<TEntity, TReturn>(IProjection column, Junction where, int top) where TEntity : BaseEntity
{
    IQueryOver<TEntity> query = null;
    if(where == null)
        query = session.QueryOver<TEntity>().Select(column);
    else
        query = session.QueryOver<TEntity>().Select(column).Where(where);

    IList<TReturn> instance = null;
    if(top == 0)
        instance = query.List<TReturn>();
    else
        instance = query.Take(top).List<TReturn>();
    return instance;
}

TEntity in above code is entity that represent (mapped to) your table. Note that this is just to build the query. It will NOT return Entity.

TReturn is the return type. This can be any standard data type like int in your case.

IProjection column parameter is the name of the column you want to select.

Junction where parameter allows you to specify the filter on rows if any. To retrieve all rows, pass it null .

Following is the way you call it:

Junction where = Restrictions.Conjunction();
where.Add(Restrictions.Eq(..........));

IList<int> idList = GetValues<SomeTableEntity, int>(Projections.Property<SomeTableEntity>(x => x.Id), where, 0);

This way, you avoid writing hard-coded SQL query as string in your code. As you can see, this function can be used with any Entity (table) and any column.