Allen Bierbaum wrote:
> 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?
MappedCollection doesn't currently have a mismatch guard on __setitem__
(d[key] = val) or setdefault(), but easily could. There *is* a guard
protecting against item.notes = {'not-color': Note('color', 'blue')}, so
that machinery is available and applying it to the other setters is
straightforward.
Automatically setting the value for the attribute_ and column_mapped
dict collections would be pretty convenient and DRY. This is a great
time to integrate that feature, if you want to try your hand at putting
together a patch and tests. If it's not too disruptive to existing
users it could slide right in as a new feature of 0.5.
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