Question

I’m using fluent nhibernate for the first time to phase out my existing database access layer for accessing a remote MySql server.

The clients are mostly virtualized and can go to sleep/hibernate at any point in time, for any length of time (seconds to days). Even though they are usually time synchronous with the server that cannot be guaranteed, which means I cannot use the client provided DateTime for some fields.

How do I get nhibernate to set/update certain fields with the server time (preferably by calling the UTC_TIMESTAMP() function)?

Object:

public class MyObject{
  public virtual UInt64 Id { get; set; }
  public virtual String Status{ get; set; }
  public virtual String SomeData{ get; set; }

  //Set only once when the Object is inserted
  public virtual DateTime TimeCreated { get; set; }
  //Set every time the object is updated
  public virtual DateTime TimeLastUpdate { get; set; }
  //Set every time the 'Status' column is updated
  public virtual DateTime TimeStatusLastChanged { get; set; }
  //This is a user provides standard datetime field
  public virtual DateTime SomeUserSpecifiedTime { get; set; }
}

Mapping:

Id(x => x.Id)
  .GeneratedBy.Native();
Map(x => x.Status);
Map(x => x.SomeData);
Map(x => x.SomeUserSpecifiedTime);
//? -->
Map(x => x.TimeCreated)
  .Not.Update();
Map(x => x.TimeLastUpdate);
Map(x => x.TimeStatusLastChanged);
//<-- ?

I want the three Time*-fields to use the UTC_TIMESTAMP() function, when they are set/updated, so the server side date/time gets inserted. The DateTime SomeUserSpecifiedTime field is a standard mapped field.

Until now my database access class contained the logic to create the queries in that way. I could create database triggers with the .Generated.Insert/Always mapping, but I was hoping there is a nhibernate/code-only way of doing that.

One solution I have found, is fetching the time from the server and providing it for the insert/update, but that is out, because the time between fetch and insert/update can be significant.

Answer 1

I’ve decided to solve the problem using triggers, which is not optimal, but works for now. Unfortunately nhibernate does not have a concept of triggers, so you have to declare them as auxiliary database objects, which leads to some problems

• SchemaValidator and SchemaUpdate will not pick up on auxDbObjects and thus will not update/create them. SchemaExport will create them, but will do so twice which leads to even more code duplication.
• Need to create and register an instance for each trigger, which is cumbersome.

It might look something like this

public class MyTrigger : IAuxiliaryDatabaseObject{

  public string SqlCreateString(Dialect dialect, IMapping p, string defaultCatalog, string defaultSchema){
    //The drop is important, because the shema export calls and executes this twice.
    return @"
DROP TRIGGER IF EXISTS myTrigger;

CREATE TRIGGER myTrigger BEFORE INSERT ON myObjectTable FOR EACH ROW 
BEGIN
set new.TimeAdded = UTC_TIMESTAMP();
END";
    }

    public string SqlDropString(Dialect dialect, string defaultCatalog, string defaultSchema){
    return @"DROP TRIGGER IF EXISTS myTrigger";
    }

  public void AddDialectScope(string dialectName){
    throw new NotImplementedException();
    }

  public bool AppliesToDialect(Dialect dialect){
    return true;
    }

  public void SetParameterValues(IDictionary<string, string> parameters){
    throw new NotImplementedException();
    }
  }

In the configuration each trigger/aux. database object must be added individually:

Fluently.Configure()
  .Database(...)
  .ExposeConfiguration(conf =>
    {
    conf.AddAuxiliaryDatabaseObject(new MyTrigger());
    });

The mapping

Map(x => x.TimeCreated)
  .Not.Update()
  .CustomType("UtcDateTime")
  .ReadOnly()
  .Generated.Insert()   //.Generated.Always() for columns that track lastModification
  ;

Custom user types or interceptors do not work unfortunately so it seems this is the only possible way to achieve this at the moment.