Source code for examples.association.dict_of_sets_with_default
"""An advanced association proxy example which
illustrates nesting of association proxies to produce multi-level Python
collections, in this case a dictionary with string keys and sets of integers
as values, which conceal the underlying mapped classes.
This is a three table model which represents a parent table referencing a
dictionary of string keys and sets as values, where each set stores a
collection of integers. The association proxy extension is used to hide the
details of this persistence. The dictionary also generates new collections
upon access of a non-existent key, in the same manner as Python's
"collections.defaultdict" object.
"""
import operator
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import String
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session
from sqlalchemy.orm.collections import KeyFuncDict
class Base:
id = Column(Integer, primary_key=True)
Base = declarative_base(cls=Base)
class GenDefaultCollection(KeyFuncDict):
def __missing__(self, key):
self[key] = b = B(key)
return b
class A(Base):
__tablename__ = "a"
associations = relationship(
"B",
collection_class=lambda: GenDefaultCollection(
operator.attrgetter("key")
),
)
collections = association_proxy("associations", "values")
"""Bridge the association from 'associations' over to the 'values'
association proxy of B.
"""
class B(Base):
__tablename__ = "b"
a_id = Column(Integer, ForeignKey("a.id"), nullable=False)
elements = relationship("C", collection_class=set)
key = Column(String)
values = association_proxy("elements", "value")
"""Bridge the association from 'elements' over to the
'value' element of C."""
def __init__(self, key, values=None):
self.key = key
if values:
self.values = values
class C(Base):
__tablename__ = "c"
b_id = Column(Integer, ForeignKey("b.id"), nullable=False)
value = Column(Integer)
def __init__(self, value):
self.value = value
if __name__ == "__main__":
engine = create_engine("sqlite://", echo=True)
Base.metadata.create_all(engine)
session = Session(engine)
# only "A" is referenced explicitly. Using "collections",
# we deal with a dict of key/sets of integers directly.
session.add_all([A(collections={"1": {1, 2, 3}})])
session.commit()
a1 = session.query(A).first()
print(a1.collections["1"])
a1.collections["1"].add(4)
session.commit()
a1.collections["2"].update([7, 8, 9])
session.commit()
print(a1.collections["2"])