URLFor and many relationships

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='',
#),
'collection': ma.URLFor('lease_v1_bp.get_leases'),
}
```

Start with the LeaseSchema definition, especially the nested resources field and the commented ma.URLFor configuration, then trace how the lease_v1_bp endpoint receives URL parameters. Determine whether URLFor supports deriving a discriminator from a list relationship, and document or implement a clear supported approach whose generated self URL matches the requested resource.

Written by the indexing model from the issue text.