Dictionary of tags in declarative SQLAlchemy?

Question

I am working on a quite large code base that has been implemented using sqlalchemy.ext.declarative, and I need to add a dict-like property to one of the classes. What I need is the same as in this question, but in a declarative fashion. Can anyone with more knowledge in SQLAlchemy give me an example? Thanks in advance...

Answer 1

Declarative is just another way of defining things. Virtually you end up with the exact same environment than if you used separated mapping.

Since I answered the other question, I'll try this one as well. Hope it gives more upvotes ;)

Well, first we define the classes

from sqlalchemy import Column, Integer, String, Table, create_engine
from sqlalchemy import orm, MetaData, Column, ForeignKey
from sqlalchemy.orm import relation, mapper, sessionmaker
from sqlalchemy.orm.collections import column_mapped_collection
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('sqlite:///:memory:', echo=True)
Base = declarative_base(bind=engine)

class Note(Base):
    __tablename__ = 'notes'

    id_item = Column(Integer, ForeignKey('items.id'), primary_key=True)
    name = Column(String(20), primary_key=True)
    value = Column(String(100))

    def __init__(self, name, value):
        self.name = name
        self.value = value        

class Item(Base):
    __tablename__ = 'items'
    id = Column(Integer, primary_key=True)
    name = Column(String(20))
    description = Column(String(100))
    _notesdict = relation(Note, 
                          collection_class=column_mapped_collection(Note.name))
    notes = association_proxy('_notesdict', 'value', creator=Note)

    def __init__(self, name, description=''):
        self.name = name
        self.description = description

Base.metadata.create_all()

Now let's make a test:

Session = sessionmaker(bind=engine)
s = Session()

i = Item('ball', 'A round full ball')
i.notes['color'] = 'orange'
i.notes['size'] = 'big'
i.notes['data'] = 'none'

s.add(i)
s.commit()
print i.notes

I get:

{u'color': u'orange', u'data': u'none', u'size': u'big'}

Now let's check the notes table...

for note in s.query(Note):
    print note.id_item, note.name, note.value

I get:

1 color orange
1 data none
1 size big

It works!! :D