Duplicate constraint on many-to-many

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If I have a many-to-many setup (Person, Role, PersonRole link table):

    Person->has_many( roles => [ PersonRole => 'role' ] );
    Role->has_many( people => [ PersonRole => 'person' ] );

I can make a $person and 'Admin':

    $person->add_to_roles( {
        role    => Role->find_or_create( { name => 'Admin' } )
    } );

If this gets called again on the same $person I'd like it to quietly
fail -- that is, not add in a duplicate relationship in the link
table.

I can (and do) add a CONSTRAINT in the database:

    create table person_role (
        id      SERIAL PRIMARY KEY,
        role    integer NOT NULL REFERENCES role,
        person  integer NOT NULL REFERENCES person,
        CONSTRAINT key UNIQUE (role, person)
    );

but then I need to trap the error.  Saying someone is an Admin when
they already are an admin isn't really an error.

I thought I might be able to use $PersonRole->add_constraint and use
$changing to search for an existing record. But $changing looks
different when $role->add_to_people is called than it does when
$person->add_to_roles is called.  I suppose I can just test for both
cases.

Must be a common issue, so is there a standard way to test for an
existing relationship?

Make a new method that does this?

    $person->add_to_roles( { role = $role } )
        unless grep { $role = $_ } $person->roles;




        -- 
        Bill Moseley
moseley@xxxx.xxx