NHibernate什么时候执行我的查询？

Question

I am trying to write a generic repository for my NHibernate data access. The Get<T>() method should be able to take an optional predicate, that should be included in the query - that is, NHibernate should generate the WHERE clause in the SQL.

    public virtual IList<T> Get(Func<T, bool> predicate = null)
    {
        // Open NHibernate Session
        using (var session = NHibernateHelper.OpenSession())
            return (predicate != null
                       ? session.Query<T>().Where(predicate)
                       : session.Query<T>()).ToList();

    }

When I pass in a predicate, and observe the SQL statement NH generates, I see no where clause.

When does NHibernate execute the query? Right when calling .Query<T>() ? If so, how can I achieve this?

Answer 1

The query should be executed by the call ToList() .

The case why the WHERE clause is not included in your sql statement is that you need to pass an Expression<Func<T,bool>> to your method.

public virtual IList<T> Get(Expression<Func<T, bool>> predicate = null)
    {
        // Open NHibernate Session
        using (var session = NHibernateHelper.OpenSession())
            return (predicate != null
                       ? session.Query<T>().Where(predicate)
                       : session.Query<T>()).ToList();

    }

The extension method Where(Func<T,bool>>) is defined on Enumerable , so that the the query loads all data and then applies the WHERE-filter in memory.

The extension method Where(Expression<Func<T,bool>>) is defined on Queryable , so that the query provider (NHibernate) can build a sql statement including your WHERE condition that gets executed on the data source.

Answer 2

Since @Jehof gave you correct explanation I just want to add separate note - you should not return IList<T> from you repository method as then any other linq operation will be executed in memory and not in the database. Assume following calls

var data = repository.Get<Company>(c=>c.Name.StartsWith("MyCompany"));
... some other operations / method calls etc.
var companySubset = data.Where(...);

so now if you have IList<T> Get<T>() you decrease performance but with IQueryable<T> Get<T> you would still have the second Where() appended to the database query.

Of course not all linq operations are supported by IQueryable like (join , last ) and this is the only place to call ToList() extension to evaluate expression.