Why NHibernate updates DB on commit of read-only transaction

http://www.zvolkov.com/clog/2009/07/09/why-nhibernate-updates-db-on-commit-of-read-only-transaction/

Always be careful with NULLable fields whenever you deal with NHibernate. If your field is NULLable in DB, make sure corresponding .NET class uses Nullable type too. Otherwise, all kinds of weird things will happen. The symptom is usually will be that NHibernate will try to update the record in DB, even though you have not changed any fields since you read the entity from the database.

The following sequence explains why this happens:

1. NHibernate retrieves raw entity's data from DB using ADO.NET
2. NHibernate constructs the entity and sets its properties
3. If DB field contained NULL the property will be set to the defaul value for its type:
   • properties of reference types will be set to null
   • properties of integer and floating point types will be set to 0
   • properties of boolean type will be set to false
   • properties of DateTime type will be set to DateTime.MinValue
   • etc.
4. Now, when transaction is committed, NHibernate compares the value of
   the property to the original field value it read form DB, and since the
   field contained NULL but the property contains a non-null value,
   NHibernate considers the property dirty, and forces an update of the
   enity.

Not only this hurts performance (you get
extra round-trip to DB and extra update every time you retrieve the
entity) but it also may cause hard to troubleshoot errors with DateTime
columns. Indeed, when DateTime property is initialized to its default
value it's set to 1/1/0001. When this value is saved to DB, ADO.NET's
SqlClient can't convert it to a valid SqlDateTime value since the
smallest possible SqlDateTime is 1/1/1753!!! The exception it throws
looks like this:

NHibernate.Event.Default.AbstractFlushingEventListener - Could not synchronize database state with session 

NHibernate.HibernateException:
An exception occurred when executing batch queries --->
System.Data.SqlTypes.SqlTypeException: SqlDateTime overflow. Must be
between 1/1/1753 12:00:00 AM and 12/31/9999 11:59:59 PM. 

at System.Data.SqlTypes.SqlDateTime.FromTimeSpan(TimeSpan value) 

   at System.Data.SqlTypes.SqlDateTime.FromDateTime(DateTime value)

   at System.Data.SqlClient.MetaType.FromDateTime(DateTime dateTime, Byte cb)

   at
System.Data.SqlClient.TdsParser.WriteValue(Object value, MetaType type,
Byte scale, Int32 actualLength, Int32 encodingByteSize, Int32 offset,
TdsParserStateObject stateObj)

   at
System.Data.SqlClient.TdsParser.TdsExecuteRPC(_SqlRPC[] rpcArray, Int32
timeout, Boolean inSchema, SqlNotificationRequest notificationRequest,
TdsParserStateObject stateObj, Boolean isCommandProc)

   at
System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior
cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean
async)

   at
System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior
cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String
method, DbAsyncResult result)

   at System.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(DbAsyncResult result, String methodName, Boolean sendToPipe)

   at System.Data.SqlClient.SqlCommand.ExecuteNonQuery()

   at System.Data.SqlClient.SqlCommandSet.ExecuteNonQuery()

   at NHibernate.AdoNet.SqlClientSqlCommandSet.ExecuteNonQuery()

   --- End of inner exception stack trace ---

   at NHibernate.AdoNet.SqlClientSqlCommandSet.ExecuteNonQuery()

   at NHibernate.AdoNet.SqlClientBatchingBatcher.DoExecuteBatch(IDbCommand ps)

   at NHibernate.AdoNet.AbstractBatcher.ExecuteBatch()

   at NHibernate.Engine.ActionQueue.ExecuteActions(IList list)

   at NHibernate.Engine.ActionQueue.ExecuteActions()

   at NHibernate.Event.Default.AbstractFlushingEventListener.PerformExecutions(IEventSource session)

   at NHibernate.Event.Default.DefaultFlushEventListener.OnFlush(FlushEvent event)

   at NHibernate.Impl.SessionImpl.Flush()

   at NHibernate.Transaction.AdoTransaction.Commit()

The easiest fix is to make the class
property use Nullable<T> type, in this case "DateTime?".
Alternatively, you could implement a custom type mapper by implementing
IUserType with its Equals method properly comparing DbNull.Value with
whatever default value of your value type. In our case Equals would need
to return true when comparing 1/1/0001 with DbNull.Value. Implementing a
full-functional IUserType is not really that hard but it does require
knowledge of NHibernate trivia so prepare to do some substantial
googling if you choose to go that way.

Hope this helps somebody!