Return relationship fields of a schema

    :param Schema schema: a marshmallow schema
    :param list: list of relationship fields of a schema

def get_relationships(schema, model_field=False):
    """Return relationship fields of a schema

    :param Schema schema: a marshmallow schema
    :param list: list of relationship fields of a schema
    """
    relationships = [key for (key, value) in schema._declared_fields.items() if isinstance(value, Relationship)]

    if model_field is True:
        relationships = [get_model_field(schema, key) for key in relationships]

    return relationships