A fix for SqlalchemyDataLayer for flask-rest-jsonapi that applies multiple relationships without Sqlalchemy errors

data_layer.py

from flask_rest_jsonapi.data_layers.alchemy import SqlalchemyDataLayer
from flask_rest_jsonapi.schema import get_relationships


class ImprovedRelationshipsDataLayer(SqlalchemyDataLayer):
    """
    Improves SqlalchemyDataLayer by setting multiple
    relationships without Sqlalchemy errors
    """

    def apply_relationships(self, data, obj):
        """Apply relationship provided by data to obj
        :param dict data: data provided by the client
        :param DeclarativeMeta obj: the sqlalchemy object to plug relationships to
        :return boolean: True if relationship have changed else False
        """
        relationships_to_apply = []
        relationship_fields = get_relationships(self.resource.schema)
        for key, value in data.items():
            if key in relationship_fields:
                related_model = getattr(obj.__class__, key).property.mapper.class_
                related_id_field = self.resource.schema._declared_fields[
                    relationship_fields[key]].id_field

                if isinstance(value, list):
                    related_objects = []

                    for identifier in value:
                        related_object = self.get_related_object(
                            related_model, related_id_field, {'id': identifier}
                        )
                        related_objects.append(related_object)

                    # setattr(obj, key, related_objects)
                    relationships_to_apply.append({
                        'field': key,
                        'value': related_objects
                    })
                else:
                    related_object = None

                    if value is not None:
                        related_object = self.get_related_object(
                            related_model, related_id_field, {'id': value}
                        )

                    # setattr(obj, key, related_object)
                    relationships_to_apply.append({
                        'field': key,
                        'value': related_object
                    })

        # Wait to set relationships until all have been fetched from the db
        for relationship in relationships_to_apply:
            setattr(obj, relationship['field'], relationship['value'])