Thursday, July 30, 2009

NHibernate, Oracle & Clobs

If you are starting to work with NHibernate and Oracle you will definitely go through a lengthy stressful process of denile, understanding and acceptance.

So here I will share a bit of knowledge that will make that process a bit less stressful.

If you are working with clobs/nclobs and oracle you may or may not have encountered the following error when using the microsoft provided

ORA-01461: can bind a LONG value only for insert into a LONG column

This error is not very helpful and goggling it will most likely result in topics regarding oracle patches and the like. In reality this is a bug with the microsoft oracle client driver. The driver mistakenly infers the column type of the string being saved, and tries forcing the server to update a LONG value into a CLOB/NCLOB column type. The reason for the incorrect behavior is even more obscure and only happens when all the following conditions are met.

1. when we set the IDbDataParameter.Value = (string whose length is : 4000 > length > 2000 )
2. when we set the IDbDataParameter.DbType = DbType.String
3. when DB Column is of type NCLOB/CLOB

Unfortunately NHibernate 2.0's default behavior is to do exactly the above, making it quite more likely to run into this ugly bug when using nhibernate and oracle.

It is also unfortunate that the issue has indeed been raised with the NHibernate community but has been closed as external.

I pretty much figure that since Microsoft has decided to pull their driver from the .NET framework in the future, they wont fix this external issue, so it becomes the burden of the developer to work around this ugly bug. So without much further ado here is the way I've gotten around fixing this issue.

I created a custom driver and inherited from the Oracle client driver where I override as follows.

/// <summary>
Initializes the parameter.
/// </summary>
<param name="dbParam">The db param.
<param name="name">The name.
<param name="sqlType">Type of the SQL.
protected override void InitializeParameter(System.Data.IDbDataParameter dbParam, string name, global::NHibernate.SqlTypes.SqlType sqlType)
base.InitializeParameter(dbParam, name, sqlType);

//System.Data.OracleClient.dll driver generates an exception
//we set the IDbDataParameter.Value = (string whose length: 4000 > length > 2000 )
//when we set the IDbDataParameter.DbType = DbType.String
//when DB Column is of type NCLOB/CLOB
//The Above is the default behavior for NHibernate.OracleClientDriver
//So we use the built-in StringClobSqlType to tell the driver to use the NClob Oracle type
//This will work for both NCLOB/CLOBs without issues.
//Mapping file will need to be update to use StringClob as the property type
if ((sqlType is StringClobSqlType))
((OracleParameter)dbParam).OracleType = OracleType.NClob;

I will try reopening this issue with the NHibernate commiters and see if it now warrants an update.