Question

I have question regarding the SQLAlchemy. How can I add into my mapped class the dictionary-like attribute, which maps the string keys into string values and which will be stored in the database (in the same or another table as original mapped object). I want this add support for arbitrary tags of my objects.

I found the following example in SQLAlchemy documentation:

from sqlalchemy.orm.collections import column_mapped_collection, attribute_mapped_collection, mapped_collection

mapper(Item, items_table, properties={
# key by column
'notes': relation(Note, collection_class=column_mapped_collection(notes_table.c.keyword)),
# or named attribute
'notes2': relation(Note, collection_class=attribute_mapped_collection('keyword')),
# or any callable
'notes3': relation(Note, collection_class=mapped_collection(lambda entity: entity.a + entity.b))
})

item = Item()
item.notes['color'] = Note('color', 'blue')

But I want the following behavior:

mapper(Item, items_table, properties={
# key by column
'notes': relation(...),
})

item = Item()
item.notes['color'] = 'blue'

It is possible in SQLAlchemy?

Thank you

Answer 1

The simple answer is yes.

Just use an association proxy:

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

Create a test environment:

engine = create_engine('sqlite:///:memory:', echo=True)
meta = MetaData(bind=engine)

Define the tables:

tb_items = Table('items', meta, 
        Column('id', Integer, primary_key=True), 
        Column('name', String(20)),
        Column('description', String(100)),
    )
tb_notes = Table('notes', meta, 
        Column('id_item', Integer, ForeignKey('items.id'), primary_key=True),
        Column('name', String(20), primary_key=True),
        Column('value', String(100)),
    )
meta.create_all()

Classes (note the association_proxy in the class):

class Note(object):
    def __init__(self, name, value):
        self.name = name
        self.value = value
class Item(object):
    def __init__(self, name, description=''):
        self.name = name
        self.description = description
    notes = association_proxy('_notesdict', 'value', creator=Note)

Mapping:

mapper(Note, tb_notes)
mapper(Item, tb_items, properties={
        '_notesdict': relation(Note, 
             collection_class=column_mapped_collection(tb_notes.c.name)),
    })

Then just test it:

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

That prints:

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

But, are those in the notes table?

>>> print list(tb_notes.select().execute())
[(1, u'color', u'orange'), (1, u'data', u'none'), (1, u'size', u'big')]

It works!! 🙂