load_models should support OR queries

[jace]

load_models currently treats all parameters as an AND query when loading a model. It needs the ability to perform an OR query. Consider this:

@app.route('/<channel>/<playlist>')
@load_models(
    (Channel, {'name': 'channel'}, 'channel'),
    (Playlist, {'name': 'playlist', 'channel': 'channel'}, 'playlist')
)
def playlist_view(channel, playlist):
    pass

The dictionary syntax treats all dictionary keys as part of an AND query. It should be possible to perform an OR query, checking for a match in one or more attributes. I propose this syntax:

@app.route('/<channel>/<playlist>')
@load_models(
    (Channel, {'name': 'channel'}, 'channel'),
    (Playlist, ({'name': 'playlist', 'channel': 'channel'}, {'uuid': 'playlist', 'channel': 'channel'}), 'playlist')
)
def playlist_view(channel, playlist):
    pass

Supplying a list/tuple of dictionaries should signal an OR query.

[jace]

Alternate syntax for nested AND/OR clauses:

@app.route('/<channel>/<playlist>')
@load_models(
    (Channel, {'name': 'channel'}, 'channel'),
    (Playlist, {('name', 'uuid'): 'playlist', 'channel': 'channel'}, 'playlist')
)
def playlist_view(channel, playlist):
    pass

This is possible because tuples can be dictionary keys.

Both variations of the OR syntax introduce a new problem: we currently apply the filter using filter_by, which takes string names. Using or_ (and and_) will require using filter, which requires Python expressions. This means a previously simple call:

column = 'name'

Model.query.filter_by(**{column: value})

Now looks like this

column = 'name'

Model.query.filter(getattr(Model, column) == value)

[jace]

This getattr call is in fact what the filter_by method does. It's just syntactic sugar for it.

[jace]

This is now deprecated with ClassView ( #167), where the class can specify custom loaders. That will be far more efficient than load_models supporting additional syntaxes.