Support for "join" requests resulting in mapped objects

[mfranck]

class Person;

class Book : public table< Book > {
public:
  TABLE_INIT(Book);
  column< int > id;
  column< std::string > title;
  column< std::string > author;
reference< Person > owner;
  COLLECTION(Person, borrowers);
};

has_and_belongs_to_many(Book,Person,borrowers,"borrows","borrowed_book_id", "borrower_id");

class Person : public table< Person > {
public:
  TABLE_INIT(Person);
  column< int > id;
  column< std::string > name;
  COLLECTION(Book, borrowed_books);
};

has_and_belongs_to_many(Person,Book,borrowed_books,"borrows","borrower_id","borrowed_book_id");

TEST(TEST_JOIN, should_find_with_criteria_on_to_one) {
  Person bob;
  bob.id=12345;
  Book of_bob_by_Tolkien;
  of_bob_by_Tolkien.owner( bob );
  of_bob_by_Tolkien.author.like( "Tolkien" );
  collection < Book >Tolkiens_of_bob;
  Tolkiens_of_bob=of_bob_by_Tolkien. find();
}

TEST(TEST_JOIN, should_find_with_criteria_on_to_manies) {
  Book of_Tolkien;
  of_Tolkien.author.like("Tolkien");
  Person who_borrowed_tolkien;
who_borrowed_tolkien.borrowed_books ( of_Tolkien ) ; // A kind of tautology...
  collection< Person > geeks;
  geeks = who_borrowed_tolkien.find();
}

TEST(TEST_JOIN, should_traverse_several_to_manies) {
  Person joe;
  joe.name.like( "joe" );
  Book read_by_joe;
  read_by_joe.borrowers( joe ); // Mean : Joe is one of the borrowers.
  Person fan_of_joe;
  fan_of_joe.borrowed_books( read_by_joe );
  collection< Person > fans_of_joe;
  fans_of_joe = fan_of_joe.find() ;
}

TEST(TEST_JOIN, should_traverse_several_to_manies_less_verbose) {
  collection< Person > fans_of_joe;
  fans_of_joe =Person().borrowed_books(Book().borrowers( Person().name.like("joe" ))).find();
}