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Signals

Signals are a mechanism to fire your piece of code (function / method) whenever given type of event happens in ormar.

To achieve this you need to register your receiver for a given type of signal for selected model(s).

Defining receivers

Given a sample model like following:

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import databases
import sqlalchemy

import ormar

database = databases.Database("sqlite:///db.sqlite")
metadata = sqlalchemy.MetaData()


class Album(ormar.Model):
    class Meta:
        tablename = "albums"
        metadata = metadata
        database = database

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=100)
    is_best_seller: bool = ormar.Boolean(default=False)
    play_count: int = ormar.Integer(default=0)

You can for example define a trigger that will set album.is_best_seller status if it will be played more than 50 times.

Import pre_update decorator, for list of currently available decorators/ signals check below.

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from ormar import pre_update


@pre_update(Album)
async def before_update(sender, instance, **kwargs):
    if instance.play_count > 50 and not instance.is_best_seller:
        instance.is_best_seller = True


# here album.play_count ans is_best_seller get default values
album = await Album.objects.create(name="Venice")
assert not album.is_best_seller
assert album.play_count == 0

album.play_count = 30
# here a trigger is called but play_count is too low
await album.update()
assert not album.is_best_seller

album.play_count = 60
await album.update()
assert album.is_best_seller

Define your function.

Note that each receiver function:

  • has to be callable
  • has to accept first sender argument that receives the class of sending object
  • has to accept **kwargs argument as the parameters send in each ormar.Signal can change at any time so your function has to serve them.
  • has to be async cause callbacks are gathered and awaited.

pre_update currently sends only one argument apart from sender and it's instance one.

Note how pre_update decorator accepts a senders argument that can be a single model or a list of models, for which you want to run the signal receiver.

Currently there is no way to set signal for all models at once without explicitly passing them all into registration of receiver.

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from ormar import pre_update


@pre_update(Album)
async def before_update(sender, instance, **kwargs):
    if instance.play_count > 50 and not instance.is_best_seller:
        instance.is_best_seller = True


# here album.play_count ans is_best_seller get default values
album = await Album.objects.create(name="Venice")
assert not album.is_best_seller
assert album.play_count == 0

album.play_count = 30
# here a trigger is called but play_count is too low
await album.update()
assert not album.is_best_seller

album.play_count = 60
await album.update()
assert album.is_best_seller

Note

Note that receivers are defined on a class level -> so even if you connect/disconnect function through instance it will run/ stop running for all operations on that ormar.Model class.

Note that our newly created function has instance and class of the instance so you can easily run database queries inside your receivers if you want to.

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from ormar import pre_update


@pre_update(Album)
async def before_update(sender, instance, **kwargs):
    if instance.play_count > 50 and not instance.is_best_seller:
        instance.is_best_seller = True


# here album.play_count ans is_best_seller get default values
album = await Album.objects.create(name="Venice")
assert not album.is_best_seller
assert album.play_count == 0

album.play_count = 30
# here a trigger is called but play_count is too low
await album.update()
assert not album.is_best_seller

album.play_count = 60
await album.update()
assert album.is_best_seller

You can define same receiver for multiple models at once by passing a list of models to signal decorator.

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# define a dummy debug function
@pre_update([Album, Track])
async def before_update(sender, instance, **kwargs):
    print(f"{sender.get_name()}: {instance.json()}: {kwargs}")

Of course you can also create multiple functions for the same signal and model. Each of them will run at each signal.

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@pre_update(Album)
async def before_update(sender, instance, **kwargs):
    print(f"{sender.get_name()}: {instance.json()}: {kwargs}")

@pre_update(Album)
async def before_update2(sender, instance, **kwargs):
    print(f'About to update {sender.get_name()} with pk: {instance.pk}')

Note that ormar decorators are the syntactic sugar, you can directly connect your function or method for given signal for given model. Connect accept only one parameter - your receiver function / method.

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class AlbumAuditor:
    def __init__(self):
        self.event_type = "ALBUM_INSTANCE"

    async def before_save(self, sender, instance, **kwargs):
        await AuditLog(
            event_type=f"{self.event_type}_SAVE", event_log=instance.json()
        ).save()

auditor = AlbumAuditor()
pre_save(Album)(auditor.before_save)
# call above has same result like the one below
Album.Meta.signals.pre_save.connect(auditor.before_save)
# signals are also exposed on instance
album = Album(name='Miami')
album.signals.pre_save.connect(auditor.before_save)

Warning

Note that signals keep the reference to your receiver (not a weakref) so keep that in mind to avoid circular references.

Disconnecting the receivers

To disconnect the receiver and stop it for running for given model you need to disconnect it.

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@pre_update(Album)
async def before_update(sender, instance, **kwargs):
    if instance.play_count > 50 and not instance.is_best_seller:
        instance.is_best_seller = True

# disconnect given function from signal for given Model
Album.Meta.signals.pre_save.disconnect(before_save)
# signals are also exposed on instance
album = Album(name='Miami')
album.signals.pre_save.disconnect(before_save)

Available signals

Warning

Note that signals are not send for:

  • bulk operations (QuerySet.bulk_create and QuerySet.bulk_update) as they are designed for speed.

  • queyset table level operations (QuerySet.update and QuerySet.delete) as they run on the underlying tables (more lak raw sql update/delete operations) and do not have specific instance.

pre_save

pre_save(sender: Type["Model"], instance: "Model")

Send for Model.save() and Model.objects.create() methods.

sender is a ormar.Model class and instance is the model to be saved.

post_save

post_save(sender: Type["Model"], instance: "Model")

Send for Model.save() and Model.objects.create() methods.

sender is a ormar.Model class and instance is the model that was saved.

pre_update

pre_update(sender: Type["Model"], instance: "Model")

Send for Model.update() method.

sender is a ormar.Model class and instance is the model to be updated.

post_update

post_update(sender: Type["Model"], instance: "Model")

Send for Model.update() method.

sender is a ormar.Model class and instance is the model that was updated.

pre_delete

pre_delete(sender: Type["Model"], instance: "Model")

Send for Model.save() and Model.objects.create() methods.

sender is a ormar.Model class and instance is the model to be deleted.

post_delete

post_delete(sender: Type["Model"], instance: "Model")

Send for Model.update() method.

sender is a ormar.Model class and instance is the model that was deleted.

pre_relation_add

pre_relation_add(sender: Type["Model"], instance: "Model", child: "Model", relation_name: str, passed_args: Dict)

Send for Model.relation_name.add() method for ManyToMany relations and reverse side of ForeignKey relation.

sender - sender class, instance - instance to which related model is added, child - model being added, relation_name - name of the relation to which child is added, for add signals also passed_kwargs - dict of kwargs passed to add()

post_relation_add

post_relation_add(sender: Type["Model"], instance: "Model", child: "Model", relation_name: str, passed_args: Dict)

Send for Model.relation_name.add() method for ManyToMany relations and reverse side of ForeignKey relation.

sender - sender class, instance - instance to which related model is added, child - model being added, relation_name - name of the relation to which child is added, for add signals also passed_kwargs - dict of kwargs passed to add()

pre_relation_remove

pre_relation_remove(sender: Type["Model"], instance: "Model", child: "Model", relation_name: str)

Send for Model.relation_name.remove() method for ManyToMany relations and reverse side of ForeignKey relation.

sender - sender class, instance - instance to which related model is added, child - model being added, relation_name - name of the relation to which child is added.

post_relation_remove

post_relation_remove(sender: Type["Model"], instance: "Model", child: "Model", relation_name: str, passed_args: Dict)

Send for Model.relation_name.remove() method for ManyToMany relations and reverse side of ForeignKey relation.

sender - sender class, instance - instance to which related model is added, child - model being added, relation_name - name of the relation to which child is added.

Defining your own signals

Note that you can create your own signals although you will have to send them manually in your code or subclass ormar.Model and trigger your signals there.

Creating new signal is super easy. Following example will set a new signal with name your_custom_signal.

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import databases
import sqlalchemy

import ormar

database = databases.Database("sqlite:///db.sqlite")
metadata = sqlalchemy.MetaData()


class Album(ormar.Model):
    class Meta:
        tablename = "albums"
        metadata = metadata
        database = database

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=100)
    is_best_seller: bool = ormar.Boolean(default=False)
    play_count: int = ormar.Integer(default=0)

Album.Meta.signals.your_custom_signal = ormar.Signal()
Album.Meta.signals.your_custom_signal.connect(your_receiver_name)

Actually under the hood signal is a SignalEmitter instance that keeps a dictionary of know signals, and allows you to access them as attributes. When you try to access a signal that does not exist SignalEmitter will create one for you.

So example above can be simplified to. The Signal will be created for you.

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Album.Meta.signals.your_custom_signal.connect(your_receiver_name)

Now to trigger this signal you need to call send method of the Signal.

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await Album.Meta.signals.your_custom_signal.send(sender=Album)

Note that sender is the only required parameter and it should be ormar Model class.

Additional parameters have to be passed as keyword arguments.

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await Album.Meta.signals.your_custom_signal.send(sender=Album, my_param=True)