Using query string parameters, the expandables mixins allow you to dynamically and efficiently control the expansion of relationships in the objects returned from your views. Both the ExpandableViewMixin
and ExpandableSchemaMixin
can be used independently but are most effective when used together.
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 that expand relationships for each object returned from your view. See the expandables API <api-expandables.ExpandableViewMixin-label>
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 performs a single query for all of the books related to the authors returned by the view, which prevents performing individual quries to retrieve the books for each author returned when the authors are serialized.
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. That dictionary's keys should match both the name of the model's relationship being expanded, and the query string parameter in from the request. The dictionary's values 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')
}