Question

Is it possible to generate a schema of a database from nHibernate, where I have provided nHibernate with the configuration to the database but I have not written any mappings.

I wish to get the Database MetaData/Schema programmatically.

I am using an Oracle Database. I have tried two approaches:

Approach one:

    public DatabaseMetadata GetMetadata(DbConnection connectionIn)
    {
        return new DatabaseMetadata(connectionIn, _dialect);
    }

Problem: This seems to be what I need however, although it correctly connects, it hasn’t picked up any of my tables. All I provided was the nHibernate Configuration object which was populated with the contents of my nHibernate.xml.config file (connection string, driver client, etc).

Question: Why would it not return the table data? It’s connected correctly but finds nothing!

Approach two:

 public void DatabaseSchema()
 {
     var schema = new SchemaExport(nHibernateConfiguration);
     schema.SetOutputFile("schema.dll");
      schema.Create(true, true);
 }

nHibernateConfiguration is an instance (property on class) of the nHibernate Configuration object, populated with contents from the nHibernate.xml.config class.

Problem: This simply doesn’t work. Crashes with the following exception:

NHibernate.MappingException : Dialect
does not support identity key
generation

I suspect this will only generate a schema based on mappings you have created? I have created no mappings. The idea is this will work against whichever database I have connected to a generate a schema for it.

Question: Is my belief that this method will only generate a Schema based on my mappings? If not, Am I using it correctly?

Hopefully this is clear enough, comment if I need to provide more info.

Thanks In Advance.

To be clear: I have a database and want to get meta data representing the database, a schema.

Answer 1

NHibernate is actually based on the mapping files. You could generate classes or tables from them. There are tools to generate the mapping files, but they are based on the classes, not the tables.

Answers to your specific questions:

Approach one: NHibernate does not read table definitions from the database. All the table definitions need to be specified in the mapping files.

Approach two: SchemaExport creates an SQL file (Create tables, indexes etc) from the mapping definitions. It is actually recommended to use it, unless you need to cope with legacy databases. The output file should be called *.sql, not *.dll.

The error you get is most probably because you try to create an identity id on an oracle database (or another which does not support identity columns). Use hilo instead (or, if you don’t like it, guid.comb or native). I just wonder why you get this error, I thought that you didn’t write any mapping files?

Conclusion:

I don’t know of any tool which create NHibernate mapping files from database tables. There may be one, most probably it is not free or not mature (because otherwise it would be well known). So I suggest to think about generating the table definitions instead, or, in case you have a legacy database, you need to go through writing the mapping files manually.