Treating Db Tables As Classes?

I'm still trying to transition from a DB-centric view to an OO view,
and I could use some help from any of you OO-design wizards out there.

A very common db representation is Table A, Table B, and Table A_B, which maps the two in a one-to-many relation. E.g.,

create table Companies (
 companyId int,
 name varchar
);

create table Products (
 productId int,
 name varchar
);

create table Companies_Products (
 companyId int,
 productId int
);

So how does this translate into object-land? I started with:

public class Product
 public id as integer
 public name as string
end class

public clas Company
 public id as integer
 public name as string
 public products as generic.list(of Product)
end class

OK,
so far, so good -- I've represented most of my data. But I've lost some
important information: given a Product object, I have no easy way to
find out which company sells it.

SQL: scan the Companies_Products table
OO: iterate through companies, nested iteration through company.products.

Do I just add a function to my data access layer that does the lookup? Is that allowed?

On
a side note: can anyone recommend a good intro to OO design
specifically for database programmers? It seems like most of my
confusion comes from issues of translating tables into classes.

Thanks!