Class::DBI: Filtering a many-to-many mapping does not work as advertised on CDBI manpage?

unlinker has asked for the wisdom of the Perl Monks concerning the following question:

Venerable Monks
I want to implement a simple many-to-many relation with Class::DBI. Here is a simplified version of my scheme/objects:

package Hotels;

__PACKAGE__->table('hotels');

__PACKAGE__->columns(Primary => 'hotel_id');
__PACKAGE__->columns(Others => qw{
    name
    city
});

__PACKAGE__->has_many(categories => [HotelCategories => 'category_id']
+);

package Categories;

__PACKAGE__->table('categories');

__PACKAGE__->columns(Primary => 'category_id');
__PACKAGE__->columns(Others => qw{
    name
});

__PACKAGE__->has_many(hotels => [HotelCategories => 'hotel_id']);

package HotelCategories;

__PACKAGE__->table('hotelcategories');

__PACKAGE__->columns(Primary => 'hotelcategory_id');
__PACKAGE__->columns(Others => qw{
    hotel_id
    category_id
);

__PACKAGE__->has_a(hotel_id => Hotels);
__PACKAGE__->has_a(category_id => Categories);
[download]

I want to select all hotels of a particular category in a particular city and the Class::DBI manpage suggests the following construct:

$mycat = Categories->retrieve(3);

for ( $mycat->hotels(city => 'Timbucktoo') ) {
    # do something with hotels in Timbucktoo of category 3
}
[download]

Unfortunately this dies with an Error which in effect says: "city is not a column of HotelCategories". Of course I know this! I believe I am mimicking this specific extract from the Class::DBI manpage:

Limiting

         Music::Artist->has_many(cds => 'Music::CD');
         my @cds = $artist->cds(year => 1980);

When calling the method created by has_many, 
you can also supply any additional key/value 
pairs for restricting the search. The above 
example will only return the CDs with a year 
of 1980.
[download]

Can a kind monk point me to the error I am making? Thank you.

Comment on Class::DBI: Filtering a many-to-many mapping does not work as advertised on CDBI manpage? Select or Download Code

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Re: Class::DBI: Filtering a many-to-many mapping does not work as advertised on CDBI manpage? by perrin (Chancellor) on Nov 09, 2008 at 03:15 UTC
The example you are copying is a one-to-many, not a many-to-many with a mapping table. Class::DBI's concept of using a mapping class for many-to-many is usually a bad idea. It will result in lots of unnecessary database calls. It's much better to use the approach described as Ima::DBI methods in the man page and write set_sql() statements with joins in them.	[reply]
Re^2: Class::DBI: Filtering a many-to-many mapping does not work as advertised on CDBI manpage? by unlinker (Monk) on Nov 09, 2008 at 03:33 UTC
Thanks. I assumed that since a many-to-many was a two way one-to-many via an intersection table (in my case HotelCategories) using a one-to-many example in either direction would work. Thanks for the suggestion, I will check out the Ima::DBI methods.	[reply]
Re: Class::DBI: Filtering a many-to-many mapping does not work as advertised on CDBI manpage? by NetWallah (Canon) on Nov 09, 2008 at 00:26 UTC
I have used Class::DBI just a few times, so please excuse potentially incorrect advice. IMHO, the links should be made like this: `__PACKAGE__->has_many(hotels => [HotelCategories => 'Hotels']);` [download] Using the 'package name' instead of a specific column name for the linkage. Have you been high today? I see the nuns are gay! My brother yelled to me...I love you inside Ed - Benny Lava, by Buffalax	[reply] [d/l]
Re^2: Class::DBI: Filtering a many-to-many mapping does not work as advertised on CDBI manpage? by unlinker (Monk) on Nov 09, 2008 at 03:43 UTC
Thanks. The documentation says that the following generic template `A->has_many(bees => [AB => 'a'])` [download] will call the method 'a' on each of the objects of type AB. This is exactly what I thought I should have got. I will check your suggestion (where did you see that syntax?) and report. Thank you once again for the help	[reply] [d/l]
Re^3: Class::DBI: Filtering a many-to-many mapping does not work as advertised on CDBI manpage? by NetWallah (Canon) on Nov 09, 2008 at 18:17 UTC
The documentation for "has_many" reads: `Class->has_many(method_to_create => "Foreign::Class");` [download] This method declares that another table is referencing us (i.e. storing our primary key in its table). It creates a named accessor method in our class which returns a list of all the matching Foreign::Class objects. In addition it creates another method which allows a new associated object to be constructed, taking care of the linking automatically. This method is the same as the accessor method with ``add_to_'' prepended. The syntax that you reference has this to say: Sometimes we don't want to return an instance of the Foreign::Class, but instead the result of calling a method on that object. We can do this by changing the Foreign::Class declaration to a listref of the Foreign::Class and the method to call on that class. Which I don't fully comprehend. Have you been high today? I see the nuns are gay! My brother yelled to me...I love you inside Ed - Benny Lava, by Buffalax	[reply] [d/l]