Expandables

The expandables mixins allow you to dynamically and efficiently control the expansion of relationships in the objects returned from your views using query string parameters. Both the ExpandableViewMixin and ExpandableSchemaMixin can be used independently but are most effective when used together.

ExpandableViewMixin

The ExpandableViewMixin allows you to dynamically control the joins executed by your view for each request. Using the expandable_fields class attribute you can configure query joins and options based on the values passed into the expand query string parameter. This allows you to avoid inadvertently executing extra queries to expand relationships for each object returned from your view. See the epandables API documentation for more information on using the expandable_fields attribute.

Example:

from sqlalchemy.orm import subqueryload

from pyramid_restful import viewsets
from pyramid_restful.expandables import ExpandableViewMixin


class AuthorViewSet(viewsets.CRUDModelViewSet,
                    ExpandableViewMixin):
    model = Author
    schema_class = AuthorSchema
    expandable_fields = {
        'books': {'options': [subqueryload(Author.books)]
    }

Using the example class above, a request with a query string of ?expand=books would results in subqueryload(Author.books) being passed to the .options() method on the SQLAlchemy query executed by the view. This effectively pre-queries all the books related to the authors returned by the view so that the individual quries to retrieve the books are not executed for each author returned when the authors are serialized.

ExpandableSchemaMixin

The ExpandableSchemaMixin is a mixin class for marshmallow.Schema classes. It supports optionally including Nested fields based on the value of the query string parameters. The query string parameter’s key is determined by the value of the QUERY_KEY class attribute.

Fields that can be expanded are defined in the schema’s Meta class using the expandable_fields attribute. The value of expandable_fields should be a dictionary who’s keys are used to match the value of the request’s query string parameter and the value should be a marshmallow.fields.Nested definition.

Example:

from marshmallow import Schema, fields

from pyramid_restful.expandables import ExpandableSchemaMixin

class AuthorSchema(ExpandableSchemaMixin, schema)
    id = fields.Integer()
    name = fields.String()

    class Meta:
        expandable_fields = {
            'books': fields.Nested('BookSchema')
        }