object-relational mapping

问题:

I need to store an instance of an complex datatype into a relational database. Is there a way to do this without moddeling the database structure first, like it is done in ado.net? The database (or table) structure should be created from the class structure. The class has some properties like ints, strings or bools but could also have more complex ones. I am grateful for every helping advice...

I want to do this in c#...

Update:

Thank you for your replys. I tried "code first" of EF 4 (Thank you very much Ramesh) and got my objects into the database. But now I am having a problem to get the data out of the db back to the instance of an object. My classes look like that:

class Foo {
    public int id { get; set; }
    public string Woot { get; set; } 
}

class Bar {
    public int id { get; set; }
    public string name { get; set; }
    public ICollection<Foo> FooList { get; set; }
}

So, as I said i can create instances of these classes and write them to the db. But when I try to create new instances from the db data, just the int and the string of the "Bar"type is recovered but the Collection of Foos is emtpy. My db context looks like this:

class DBstructure: DbContext
{
    public DbSet<Foo> Foos { get; set; }
    public DbSet<Bar> Bars { get; set; }
}

any Idea?

回答1:

Yes thats possible, you have to use Entity framework 4 for this. the approach is called "Code First".

you can read lot about this here in ScottGu's post Code-First Development with Entity Framework 4

回答2:

The best is to use an ORM tool that can build the database for you from your domain model (classes).

Look at NHibernate, Entity Framework to mention a few.

回答3:

I got what I wanted using the Entity Framework 4 with "code first". To write Objects from the classes I posted above, into the databse you just have to create the objects regulary:

Foo foo = new Foo();
foo.Woot = "foo1";
Foo foo2 = new Foo();
foo2.Woot = "foo2";

Bar bar = new Bar();
bar.name = "bar1";
bar.Whee = new List<Foo>();
bar.Whee.Add(foo);
bar.Whee.Add(foo2);

Then create the DbContext object, add the other objects to this context and call saveChanges() method:

DBstructure dbconn = new DBstructure();
dbconn.Database.Connection.ConnectionString = connectionString //depends on your DB
dbconn.Bars.Add(bar);
dbconn.Database.Connection.Open();
dbconn.SaveChanges();
dbconn.Database.Connection.Close();

That creates the new database with the tables "Foos" and "Bars" and another link table to manage the relations between the entitys.

Creating objects from the database information is also quite easy. Just connect to the database via the DbContext as it was done above. Create your objects and set them to the db data:

DBstructure dbconn = new DBstructure();
dbconn.Database.Connection.ConnectionString = connString;
dbconn.Foos.ToList();  //seems that you have to do that
dbconn.Bars.ToList<Bar>();

Bar barFromDB = new Bar();
barFromDB = dbconn.Bars.First<Bar>();

That works fine in my application. Sorry for the strange class and variable names. I just copied the code from my test application. Hope that helps someone...

Grüßung Jucker!