extra constraints on has_many relationship (patch)

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I would like some comments on a small patch I have attached that will allow
extra constraints to be included in has_many relationships.  This addition can
simplify some common situations.

Say we have a standard domains and users table setup, where a domain has_many
users.

My::Domains->has_many(users => 'My::Users');

Quite often you will have a field in the users table stating whether the
account is active or not.  So to get a list of all active accounts you would do
the following:

my @users = $domain->users(active => 1);

If this is done often enough, it would be easier (and less prone to error or
ommission) if we had another method called activeusers that achieved the same
thing.

My::Domains->has_many(users => 'My::Users');
My::Domains->has_many(activeusers => 'My::Users', { constraint => [active =>
1]);

my @users = $domain->activeusers();

Now you can call ->users or ->activeusers depending on what you are after.

The included patch only adds 1 line of code to Class::DBI::Relationship::HasMany
 to implement this (which is a testament to the implementation of Class::DBI). 
It does add a bit of complexity to the has_many method, but only if one is
interested in using this feature.

Of course there are probably other ways to achieve the above results, but this
seems to me like a simple and clean way of handling a common situation.

Comments/Questions?

Cees

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