URLFor and many relationships

[retr0h]

I haven't found much luck with the following.

I'd like to use URLFor to describe a URL for a relationship. I am rendering the child model Lease, which back_populates resources.

When I am deserializing the Lease object and links is generated, the parent Resource object is desereialized as well, but since it is a list, I am unsure how I can use that relationship to generate a URL to self, which requires looking at lease.relationships[0].discriminator. See LeaseSchema at bottom.

Resource Model

class Resource(base_mixin.BaseMixin, db.Base):
    __tablename__ = 'resources'

    id = sqlalchemy.Column(sqlalchemy.Integer, primary_key=True)
    name = sqlalchemy.Column(sqlalchemy.String(32), unique=True)
    labels = sqlalchemy.Column(sqlalchemy.dialects.postgresql.JSONB)

    lease_id = sqlalchemy.Column(
        sqlalchemy.Integer, sqlalchemy.ForeignKey('leases.id')
    )
    lease = sqlalchemy.orm.relationship('Lease', back_populates='resources')

    discriminator = sqlalchemy.Column('type', sqlalchemy.String(50))

    __mapper_args__ = {'polymorphic_on': discriminator}

Sapro Model

class Sapro(resource.Resource):
    __mapper_args__ = {
        'polymorphic_identity': 'sapro',
    }

    def __init__(self, *args, **kwargs):
        super(Sapro, self).__init__(*args, **kwargs)

Lease Model

class Lease(base_mixin.BaseMixin, db.Base):
    __tablename__ = 'leases'
    id = sqlalchemy.Column(sqlalchemy.Integer, primary_key=True)

    resources = sqlalchemy.orm.relationship(
        'Resource', back_populates='lease', cascade='all,delete'
    )

Lease Schema

class LeaseSchema(ma.Schema):
    class Meta:
        fields = (
            'created_at',
            'updated_at',
            'id',
            'resources',
            'links',
        )

    resources = marshmallow.fields.Nested(sapro.SaproSchema)
    links = ma.Hyperlinks(
        {
            # Unsure how to pass the discriminator from resources to URLFor.
            #'self': ma.URLFor(
            #    'lease_v1_bp.test_get_leases_by_discriminator',
            #    resources='<resources>',
            #),
            'collection': ma.URLFor('lease_v1_bp.get_leases'),
        }