[ZODB-Dev] AdaptableStorage and OR mapping strategy (was: AdaptableStorage and ZClass instances)

[Roché Compaan]

> So one still needs to create a gateway per domain class manually (which
> I don't mind as long as it requires less work than it currently does). I
> think a very simple solution would be to have a SQLGatewayBase that have
> methods that generate SQL based on a very simple schema definition in
> subclasses. To take the current SQLProperties class as an example, it
> should only need to have to look like this:
> 
>     class SQLProperties:
> 
>         __implements__ = IGateway
> 
>         schema = RowSequenceSchema()
>         schema.addField('id', 'string', 1)
>         schema.addField('type', 'string')
>         schema.addField('data', 'string')
> 
>         table_base_name = 'properties'
> 
> This will give SQLGatewayBase more than enough to generate all the
> necessary SQL and handle load and store automatically for simple
> mappings. I'll give this a bash with my next try. 


I have started rewriting SQLGatewayBase to do more work. So far it's
going well but I wanted to bounce some ideas off you wrt the state that
the load method should return.

Here's my thinking at the moment:

    If the schema is of type FieldSchema then the state returned should
    be a single value eg.:

        items = self.execute(self.readsql(key))
        if items:
            state = items[0][0]
        else:
            state = ''
        return state, state

    If the schema is of type RowSchema then the state returned should be
    a sequence and we assert that a single row is returned:

        items = self.execute(self.readsql(key))
        if items:
            assert len(items) = 1
            state = items[0]
        else:
            state = ''
        return state, state

    If the schema is of type RowSequenceSchema then the state returned
    should be a sequence:

        items = self.execute(self.readsql(key))
        if items:
            state = items
            # to ensure a hash of the state stays the same
            state.sort()
        else:
            state = []
        return state, tuple(state)
    
If a subclass needs to do something special when computing state it can
simply override the load method.

-- 
Roché Compaan
Upfront Systems                 http://www.upfrontsystems.co.za