Models¶

Creating new models¶

Each new model should be defined as a subclass of ModelBase, and decorated with construct_nb_db_model decorator. Below we’ll introduce an example:

@model_framework.construct_nb_db_model
class Movie(model_framework.ModelBase):
    table_name = 'movies'

    title = fields.StringField(required=True)
    year = fields.IntField()
    director = fields.ReferenceField(Director)
    awards = fields.ListField(str)

The above example defines a new Movie model, that contains 5 fields:

1. id - Object identifier, derived form ModelBase, present in all model objects.
2. title - A string containing the movie title, marked as mandatory.
3. year - A year movie was published.
4. director - A reference field to an object of director type (will be covered later).
5. awards - A list of all the awards the movie received.

Class definition also contains table_name field that stores the name of the table our model is stored in the north-bound database.

Initializing this object is done by passing the values as keyword arguments.

a_space_oddyssey = Movie(
    id='movie-id-2001',
    title='2001: A Space Oddyssey',
    year=1968,
    director=Director(id='stanley-kubrick')
    awards=[
        'Academy Award for Best Visual Effects',
    ],
)

We expect to write our data to the database as JSON document, the above object will be translated to:

{
    "id": "movie-id-2001",
    "title": "2001: A Space Oddyssey",
    "year": 1968,
    "director": "stanley-kubrick",
    "awards": ["Academy Award for Best Visual Effects"]
}

Registry¶

The model framework provides another decorator, @register_model that adds the class to an internal lookup table.

This allows iterating of registered models, and retrieving models by class name or table name, e.g:

for model in iterate_models():
    instances = db_store.get_all(model)

or

movie_class = get_model('movies')
movie = movie_class(**params)

References¶

Oftentimes one model is related to another object in some manner, consider the movie example above, each movie has a director, so somewhere in our code we have a director model, in the form of:

class Director(ModelBase):
    table_name = 'directors'

    full_name = fields.StringField()

In order to allow easy association and lookup, we can define a reference field that will retrieve the actual object (by its ID) behind the scenes. Consider we have the following object in our database:

kubrick = Director(id='stanley-kubrick', full_name='Stanley Kubrick')

We how can access kubrick object through a_space_oddyssey.director, e.g.

>>> a_space_oddyssey.director.full_name
Stanley Kubrick

The fetching is done behind the scenes (first from the local cache, then from the northbound database).

Events¶

Each model can define an arbitrary set of events, which can be used to invoke callbacks on various conditions, events are inherited from parent classes, and are specified in events= parameter of construct_nb_db_model decorator:

@construct_nb_db_model(events={'premiered'})
class Director(ModelBase):
    # ...

For each event, 2 class methods are defined:

• register_{event_name}(callback) - adds callback to be invoked each time event is emmited.
• unregister_{event_name}(callback) - removes the callback from being called.

Additionally, an instance method named emit_{event_name}(*args, **kwargs) is added.

Emit can only be called on an instance, and the origin instance is passed as first parameter to all the callbacks, then, *args, and **kwargs follow. So a call

a_space_oddyssey.emit_premiered(1, 2, 3, a='a', b='b')

would be translated to a sequence of

callback(a_space_oddyssey, 1, 2, 3, a='a', b='b')

The convention of parameters is specific to each event.

The register calls can be also used as decorators for some extra syntactic sugar

@Movie.register_premiered
def on_premiere(movie):
    print('{title} has pemiered'.format(title=movie.title))

Indexes¶

To allow easy retrieval and lookup of in memory objects we use DbStore module to fetch by IDs and other properties, the new DbStore takes note of model’s indexes and creates lookups to allow faster retrieval. Indexes, similar to events are passed in indexes= parameter of construct_nb_db_model decorator and specified as a dictionary where the key is the index name and the value is the field indexed by (or a tuple of fields, if the index is multi-key). For example if we’d like to add index by year we can define it as:

@construct_nb_db_model(indexes={'by_year': 'year'})
class Director(ModelBase):
    # ...

then query db_store by providing the index and the keys:

all_1968_movies = db_store.get_all(
    Movie(year=1968),
    index=Movie.get_index('by_year'),
)

Hooks¶

We can also define several entry points to be called on various CRUD event in the north-bound API, for example, if we wished to track our access movie and director objects better we could define a common class:

class AccessMixin(MixinBase):
    created_at = fields.DateTimeField()
    updated_at = fields.DateTimeField()

    def on_create_pre(self):
        super(AccessMixin, self).on_create_pre()
        self.created_at = datetime.datetime.now()

    def on_update_pre(self):
        super(AccessMixin, self).on_update_pre()
        self.updated_at = datetime.datetime.now()

The above code updates the relevant fields on create/update operations, so if we add those as parent class in our Movie or Director classes we’ll receive the new functionality:

# ...
class Movie(ModeBase, AccessMixin):
    # ...