How to fill dict attribute with Factory-boy ?

How to fill dict attribute with Factory-boy ?

class UserFactory(factory.Factory):
    FACTORY_FOR = User

    first_name = factory.InfiniteIterator([ 'Peggie', 'Gabe', 'King', 'Fredy', 'Ole', 'Vernice' ])
    last_name = factory.InfiniteIterator([ 'Leannon', 'Barton', 'Sauer', 'Walker', 'Strosin'])
    username = factory.LazyAttribute(
        lambda o: '%s.%s@example.com' % (o.first_name.lower(), o.last_name.lower())
    )

    # How can I feed data in dict, metadata isn't another Entities (MongoDB context)
    metadata = { 
       'a': factory.InfiniteIterator(['foobar', 'foo'])
       'b': factory.InfiniteIterator(['foobar', 'foo']) 
    }

@factory.lazy_attribute_sequence
def metadata(self, seq):
    idx = seq % 2
    return {
        'a': ['foobar', 'foo'][idx],
        'b': ['foobar', 'foo'][idx],
    }

class MetadataFactory(factory.Factory):
    FACTORY_FOR = dict
    a = factory.InfiniteIterator(['foobar', 'foo'])
    b = factory.InfiniteIterator(['foobar', 'foo'])

    @classmethod
    def _prepare(cls, create, *args, **kwargs):
        # HACK: force 'build' mode
        return super(MetadataFactory, cls)._prepare(False, *args, **kwargs)

class UserFactory(factory.Factory):
    metadata = factory.SubFactory(MetadataFactory)