I have just started using column_mapped_collections.
(http://www.sqlalchemy.org/docs/04/mappers.html#advdatamapping_relation_collections_dictcollections
I must say, these are very powerful and extremely nice when reading
data. But I have run into one thing that seems confusing when it
comes to creating new objects in a session. Namely, it is possible to
add data to the mapped dictionary in such a way that the data
structure is inconsistent and not what it would be when reading the
same data back.
Using the example from the documentation as a start:
mapper(Item, items_table, properties={
'notes': relation(Note,
collection_class=column_mapped_collection(notes_table.c.keyword)),
})
# ...
item = Item()
item.notes['color'] = Note('color', 'blue') # Set keyword attribute to 'color'
print item.notes['color']
Everything is good here, but what if I did it this way instead....
item.notes['not-color'] = Note('color', 'blue')
This last line is the problem because it has inserted a link to a new
Note that has a keyword of
'color' but is showing up in the dictionary as 'not-color'. If we
flush all of this and reload from the database using a query, there
will be no 'not-color' entry in the database.
Anyway, I think this is a bit non-intuitive. What I propose instead
is that SA could automatically set the 'keyword' attribute of the Note
object as part of the process of assigning it to the mapped collection
dictionary. This way the insert could look more like:
item.notes['not-color'] = Note(value='blue')
and behind the scenes SA would call: <new Note>.keyword = 'not-color'
Any thoughts on this? Has anyone tried this in the past?
-Allen
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