[web2py] smartgrid duplicate rows in many-to-many link table - how to avoid?

[Cliff]

I am trying to keep smartgrid from creating duplicate rows in a
linking table, but it isn't working.

Here is what I'm trying to do, but it isn't working.

Any suggestions?

Model:
db.define_table('dog',
                Field('dog_name', length=32),
                auth.signature,
                format='%(dog_name)s',
                singular = 'Dog',
                plural = 'Dogs',
               )

db.define_table('owner',
                Field('name', length=32),
                auth.signature,
                format = '%(name)s',
               )

db.define_table('dog_owner',
                Field('dog_id', db.dog),
                Field('owner_id', db.owner),
               )

db.dog_owner.dog_id.requires=IS_IN_DB(
    db, 'dog.id', '%(dog_name)s',
    zero='Choose'
)
db.dog_owner.owner_id.requires=IS_IN_DB(
    db, 'owner.id', '%(name)s',
    zero='Choose',
)

++++++++++++++++++++++++++++++++++++++++++++++
Controller code intended to avoid duplicates:
++++++++++++++++++++++++++++++++++++++++++++++
# creating a new row in the dog_owner table
    for arg in request.args:
        if '.' in arg and 'new' in request.args:

# find the id of dogs, if any, that already belong to the owner
            rows = db(db.dog_owner.owner_id==request.args[-3]).select(
                db.dog_owner.dog_id
            )

# if there are such, build a list
            if len(rows) > 0:
                avoid = []
                [avoid.append(row.dog_id) for row in rows]
                constraints = {'dog': ~(db.dog.id.belongs(avoid)) }

# also tried simple constraints such as this:
# {'dog': db.dog.id < 5}

# the dog pulldown still displays all the dogs in the db
# thereby allowing the user to create duplicate rows in dog_owner
# how can I fix this?
    form = SQLFORM.smartgrid(
        db.owner,
        ui='jquery-ui',
        onupdate=lambda form: auth.archive(
            form,
            archive_current=False
        ),
        constraints=constraints
    )