为什么DBIx :: Class不能创建多对多访问器？

Question

While creating a schema from a database many-to-many relationships between tables are not created.

Is this a principal problem?

Is it possible to detect from the table structure that many-to-many relationships exist and create the respective code in schema classes automagically?

Answer 1

It is indeed a somewhat fundamental problem -- many_to_many is a " relationship bridge " and not a " relation ." The documentation explains that "the difference between a bridge and a relationship is, that the bridge cannot be used to join tables in a search, instead its component relationships must be used."

On the other hand, this means that if the real relationships are correctly discovered it should be straightforward to add the many-to-many relationships automatically: First, search for tables that have two or more has_many relationships. Then, for each pair of such relationships, create a many-to-many relationship bridge. (Of course, one might hope that DBIx::Class would do this itself.)

Answer 2

The problem with developing this kind of code is that many tables that contain multiple references are not many-to-many tables, and have multiple references for other reasons. For instance, I'll make up a schema for some fictional app where something could be regarded as a many-to-many table, when it is not.

create table category (
    id primary key,
    ...
);

create table sub_category (
    id primary key,
    category references category(id),
    ...
);

/* EDIT:
    This is the table that could be regarded as many_to_many
    by an automated system */
create table product (
    id primary key,
    category references category(id),
    sub_category references sub_category(id),
    ...
);

Something could be built this way for ease of use, without having to do multiple table joins in the database on a website, especially when considering speed. It would be difficult for a piece of code to say definitively 'this is not a many_to_many' situation, while the developer should be able to easily figure it out, and add in the many_to_many line below the checksum.

I consider DBIX::Class schema outputs a good starting point, and little more, especially when working with auto numbering in non-MySQL databases, among other things. I often need to modify above the "Don't modify above this line" stuff (although many_to_many can obviously go below that checksum, of course.