Mapping Error in NHibernate

问题

I am trying to use NHibernate to connect to a Northwind database. But for some reason, I am not able to load the Entity Type.

This is my Entity class

public class Product
    {
        public virtual Int32 ProductId { get; set; }
        public virtual String Desc { get; set; }
    }

Here is my Mapping

 <class name="Product" table="Products">
    <id name="ProductId" column="ProductId" type="Int32">
      <generator class="identity"></generator>
    </id>
    <property name="Desc" column="ProductName" type="String" length="60">
    </property>
  </class>

I am getting the following error message

could not load an entity: [OracleLinq.Product#12][SQL: SELECT product0_.ProductId as ProductId0_0_, product0_.ProductName as ProductN2_0_0_ FROM Products product0_ WHERE product0_.ProductId=?]

Here is the stack Trace

   at NHibernate.Loader.Loader.LoadEntity(ISessionImplementor session, Object id, IType identifierType, Object optionalObject, String optionalEntityName, Object optionalIdentifier, IEntityPersister persister)
   at NHibernate.Loader.Entity.AbstractEntityLoader.Load(ISessionImplementor session, Object id, Object optionalObject, Object optionalId)
   at NHibernate.Loader.Entity.AbstractEntityLoader.Load(Object id, Object optionalObject, ISessionImplementor session)
   at NHibernate.Persister.Entity.AbstractEntityPersister.Load(Object id, Object optionalObject, LockMode lockMode, ISessionImplementor session)
   at NHibernate.Event.Default.DefaultLoadEventListener.LoadFromDatasource(LoadEvent event, IEntityPersister persister, EntityKey keyToLoad, LoadType options)
   at NHibernate.Event.Default.DefaultLoadEventListener.DoLoad(LoadEvent event, IEntityPersister persister, EntityKey keyToLoad, LoadType options)
   at NHibernate.Event.Default.DefaultLoadEventListener.Load(LoadEvent event, IEntityPersister persister, EntityKey keyToLoad, LoadType options)
   at NHibernate.Event.Default.DefaultLoadEventListener.OnLoad(LoadEvent event, LoadType loadType)
   at NHibernate.Impl.SessionImpl.FireLoad(LoadEvent event, LoadType loadType)
   at NHibernate.Impl.SessionImpl.ImmediateLoad(String entityName, Object id)
   at NHibernate.Proxy.AbstractLazyInitializer.Initialize()
   at NHibernate.Proxy.AbstractLazyInitializer.GetImplementation()
   at NHibernate.Proxy.Poco.Castle.CastleLazyInitializer.Intercept(IInvocation invocation)
   at Castle.DynamicProxy.AbstractInvocation.Proceed()
   at ProductProxy4c67cf5bf6e640ab82d8c21a90e2a62b.set_Desc(String value)
   at OracleLinq.Form1.Form1_Load(Object sender, EventArgs e)

Am I doing anything wrong?

回答1:

Second try:

Here is you configuration from your comments, just for readability:

Configuration cfg = new Configuration(); 
cfg.Configure(); 
cfg.AddAssembly(typeof(Product).Assembly); 
ISessionFactory sessionFactory = cfg.BuildSessionFactory(); 

IDbConnection conn = new SqlConnection(@"Data Source=(local);Initial Catalog=Northwind;Integrated Security=SSPI;"); 
ISession session = sessionFactory.OpenSession(conn); 

Product product = (Product)session.Load(typeof(Product), 12); 
product.Desc = "";

It's clear that NHibernate has the mapping file, it could not have generated the query.

It could be that

the database is not there : you already checked this.
the table is not there : open a sql console (using the same connect string) and copy-paste the sql from the error message into it. Does it work?
the connection is not opened : see bollow

I think that you need to open the connection yourself. Even better you let NHibernate create and manage the connections.

Try this:

Configuration cfg = new Configuration(); 
cfg.Configure(); 
cfg.AddAssembly(typeof(Product).Assembly); 
ISessionFactory sessionFactory = cfg.BuildSessionFactory(); 

ISession session = sessionFactory.OpenSession(); 

Product product = (Product)session.Load(typeof(Product), 12); 
product.Desc = "";

the connectstring goes to the nhibernate.cfg.xml

<property name="connection.provider">NHibernate.Connection.DriverConnectionProvider, NHibernate</property>
<property name="connection.connection_string">
 Data Source=(local);Initial Catalog=Northwind;Integrated Security=SSPI;
</property>
<property name="dialect">NHibernate.Dialect.MsSql2000Dialect</property>

回答2:

Is your connection string defaulting to the correct database? Check that it has Initial Catalog= [DB name here]

回答3:

This should actually only be a problem when inserting, but you could also get troubles with it:

I think "identity" is not supported by oracle, it is a SqlServer feature (an auto-counting primary key). Oracle uses sequences.

Try the following (check the name of the sequence, which must exist)

<id name="ProductId" column="ProductId" type="Int32">
  <generator class="sequence">
    <param name="sequence">product_seq</param>
  </generator>
</id>

Or an other id generator. Hilo or Guid are interesting alternatives. See http://barchitect.blogspot.com/2008/07/nhibernate-generator-and-primary-key.html

回答4:

In your stacktrace, I see ProductProxy4c67cf5bf6e640ab82d8c21a90e2a62b.set_Desc(String value), which means the error occurs when you set the 'Desc' property. This is the moment NHibernate tries to load the product from the database.

In your database you don't have a product with the id you supplied to Session.Load. If you use Session.Get, you'll probably get null.

来源：https://stackoverflow.com/questions/773854/mapping-error-in-nhibernate

标签

nhibernate