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Fluent NHibernate provides an API for configuring your database, this can be used in combination with the fluent configuration or on it's own. I'm going to show some of the various ways you can use this API.

All of the following examples are best used when placed inside the Database call of the fluent configuration.

Fluently.Configure()
  .Database(/* examples here */)
  .Mappings(...)
  .BuildSessionFactory();

NHibernate has a provider and driver design for it's database interaction, these drivers handle the various different peculiarities of each database and allow NHibernate to remain agnostic; however, it does mean you need to pick the right ones for your application. This is where Fluent NHibernate comes in, we currently provide six of the most popular database providers, and wrap them up in a nice, easy to use, API.

All the database providers are located in the FluentNHibernate.Cfg.Db namespace, and are reasonably discoverable. Each configuration starts with a static property that specifies which version you want to use, if there's only one available then you should use the `Standard` property.

For example, if you're using MS SQL Server 2005, then you should use MsSqlConfiguration.MsSql2005 to begin your configuration, alternatively if you're using SQLite you can use SQLiteConfiguration.Standard.

Each configuration instance has several options available on it, some which are shared across all providers, and others that are specific to a version or database.

The most common thing you'll need to set is the connection string for the database. There are several available options for setting your connection string; if you've got it in a string you can supply that, you can have it read from an appSetting or a connectionString from the App.config, or you can build it up yourself.

The ConnectionString property on the configurations takes a lambda that alters the conection string, or a string if you're just providing a pre-prepared connection string, here are some examples:

// raw string
MsSqlConfiguration.MsSql2005
  .ConnectionString("a raw string");
 
// from appSettings
MsSqlConfiguration.MsSql2005
  .ConnectionString(c => c
    .FromAppSetting("appSettingKey"));
 
// manually constructed
MsSqlConfiguration.MsSql2005
  .ConnectionString(c => c
    .Server("my-server")
    .Database("database-name")
    .Username("jamesGregory")
    .Password("password1"));

Similar to ConnectionString is Cache which, as the name suggests, configures NHibernate's cache. It's fairly self explanatory again, so I'll just show a simple example.

MsSqlConfiguration.MsSql2005
  .Cache(c => c
    .UseQueryCache()
    .UseMinimalPuts());

This example sets the NHibernate cache.use_query_cache and cache.use_minimal_puts properties to true.

There are several other methods available in addition to ConnectionString and Cache, but they're all self explanatory. Here's an example of a couple of calls:

MsSqlConfiguration.MsSql2005
  .UseOuterJoin()
  .DefaultSchema("dbo");

Using Fluent NHibernate does not require you to sacrifice portability across RDBMSes. You can use the traditional approach to NHibernate configuration (which uses the standard .NET configuration mechanisms) to determine database drivers and connection strings this way:

var cfg = new NHibernate.Cfg.Configuration();
 
Fluently.Configure(cfg)
    .Mappings(
      m => m.FluentMappings.AddFromAssemblyOf<Entity>())
    .BuildSessionFactory();

Bringing it all together Edit

Here's an example of a complete MS SQL 2005 configuration:

MsSqlConfiguration.MsSql2005
  .ConnectionString(c => c
    .FromAppSetting("connectionString"))
  .Cache(c => c
    .UseQueryCache()
    .ProviderClass<HashtableCacheProvider>())
  .ShowSql();

When using the fluent configuration API, you would combine it like so:

Fluently.Configure()
  .Database(MsSqlConfiguration.MsSql2005
    .ConnectionString(c => c
      .FromAppSetting("connectionString"))
    .Cache(c => c
      .UseQueryCache()
      .ProviderClass<HashtableCacheProvider>())
    .ShowSql())
  .Mappings(m => m
    .FluentMappings.AddFromAssemblyOf<Entity>())
  .BuildSessionFactory();

However, if you're not using the FluentConfiguration api, then you can call either ConfigureProperties with an NHibernate Configuration instance, or ToProperties to create an IDictionary<string, string> of the properties. :)

Some shortcuts Edit

Depending on which database you're using, there may be some shortcuts available for common configurations. For example, SQLite can be used as an in-memory database, or it can be pointed at a specific file; in these cases we've provided shortcuts that build up the connection string and settings for these configurations.

SQLiteConfiguration.Standard
  .InMemory();
 
SQLiteConfiguration.Standard
  .UsingFile("database.db");

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