New API to check if a table class exists before defining it.

- Check if it's part of the metadata through a function call rather than
  checking `Base.metadata` in every single module.

- Make it possible to override them (mostly for doc generation logic
  that needs to be able to import those classes).

- Make it possible to extend them.

platypush/common/db.py

@@ -1,3 +1,6 @@
+from contextlib import contextmanager
+from dataclasses import dataclass
+
 from sqlalchemy import __version__
 
 sa_version = tuple(map(int, __version__.split('.')))
@@ -8,3 +11,38 @@ else:
     from sqlalchemy.ext.declarative import declarative_base
 
 Base = declarative_base()
+
+
+@dataclass
+class DbContext:
+    """
+    Context flags for the database session.
+    """
+
+    override_definitions: bool = False
+
+
+_ctx = DbContext()
+
+
+@contextmanager
+def override_definitions():
+    """
+    Temporarily override the definitions of the entities in the entities
+    registry.
+
+    This is useful when the entities are being imported off-context, like
+    e.g. in the `inspect` or `alembic` modules.
+    """
+    _ctx.override_definitions = True
+    yield
+    _ctx.override_definitions = False
+
+
+def is_defined(table_name: str) -> bool:
+    """
+    Check if the given entity class is defined in the entities registry.
+
+    :param table_name: Name of the table associated to the entity class.
+    """
+    return not _ctx.override_definitions and table_name in Base.metadata

platypush/entities/_base.py

@@ -30,7 +30,7 @@ from sqlalchemy.orm.exc import ObjectDeletedError
 
 import platypush
 from platypush.config import Config
-from platypush.common.db import Base
+from platypush.common.db import Base, is_defined
 from platypush.message import JSONAble, Message
 
 EntityRegistryType = Dict[str, Type['Entity']]
@@ -52,7 +52,7 @@ fail.
 logger = logging.getLogger(__name__)
 
 
-if 'entity' not in Base.metadata:
+if not is_defined('entity'):
 
     class Entity(Base):
         """

platypush/entities/acceleration.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .three_axis import ThreeAxisSensor
 
 
-if 'accelerometer' not in Base.metadata:
+if not is_defined('accelerometer'):
 
     class Accelerometer(ThreeAxisSensor):
         """
@@ -20,6 +20,7 @@ if 'accelerometer' not in Base.metadata:
             primary_key=True,
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/audio.py

@@ -1,12 +1,12 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .dimmers import Dimmer
 from .switches import Switch
 
 
-if 'volume' not in Base.metadata:
+if not is_defined('volume'):
 
     class Volume(Dimmer):
         __tablename__ = 'volume'
@@ -15,12 +15,13 @@ if 'volume' not in Base.metadata:
             Integer, ForeignKey(Dimmer.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'muted' not in Base.metadata:
+if not is_defined('muted'):
 
     class Muted(Switch):
         __tablename__ = 'muted'
@@ -29,6 +30,7 @@ if 'muted' not in Base.metadata:
             Integer, ForeignKey(Switch.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/batteries.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'battery' not in Base.metadata:
+if not is_defined('battery'):
 
     class Battery(NumericSensor):
         __tablename__ = 'battery'
@@ -19,6 +19,7 @@ if 'battery' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/bluetooth/_device.py

@@ -9,13 +9,13 @@ from sqlalchemy import (
     String,
 )
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from ..devices import Device
 from ._service import BluetoothService
 
 
-if 'bluetooth_device' not in Base.metadata:
+if not is_defined('bluetooth_device'):
 
     class BluetoothDevice(Device):
         """
@@ -68,6 +68,7 @@ if 'bluetooth_device' not in Base.metadata:
         model_id = Column(String, default=None)
         """ Device model ID. """
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/bluetooth/_service.py

@@ -8,10 +8,10 @@ from sqlalchemy import (
     String,
 )
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 from platypush.entities import Entity
 
-if 'bluetooth_service' not in Base.metadata:
+if not is_defined('bluetooth_service'):
 
     class BluetoothService(Entity):
         """
@@ -44,6 +44,7 @@ if 'bluetooth_service' not in Base.metadata:
         connected = Column(Boolean, default=False)
         """ Whether an active connection exists to this service. """
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/buttons.py

@@ -6,14 +6,14 @@ from sqlalchemy import (
     Integer,
 )
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import EnumSensor
 
 logger = logging.getLogger(__name__)
 
 
-if 'button' not in Base.metadata:
+if not is_defined('button'):
 
     class Button(EnumSensor):
         __tablename__ = 'button'
@@ -22,6 +22,7 @@ if 'button' not in Base.metadata:
             Integer, ForeignKey(EnumSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/cloud.py

@@ -6,12 +6,12 @@ from sqlalchemy import (
     String,
 )
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .devices import Device
 
 
-if 'cloud_instance' not in Base.metadata:
+if not is_defined('cloud_instance'):
 
     class CloudInstance(Device):
         """
@@ -38,6 +38,7 @@ if 'cloud_instance' not in Base.metadata:
         alerts = Column(JSON)
         backups = Column(JSON)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/contact.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import BinarySensor
 
 
-if 'contact_sensor' not in Base.metadata:
+if not is_defined('contact_sensor'):
 
     class ContactSensor(BinarySensor):
         """
@@ -18,6 +18,7 @@ if 'contact_sensor' not in Base.metadata:
             Integer, ForeignKey(BinarySensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/devices.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, Boolean, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from ._base import Entity
 
 
-if 'device' not in Base.metadata:
+if not is_defined('device'):
 
     class Device(Entity):
         """
@@ -19,6 +19,7 @@ if 'device' not in Base.metadata:
         )
         reachable = Column(Boolean, default=True)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/dimmers.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey, Float, String
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .devices import Device
 
 
-if 'dimmer' not in Base.metadata:
+if not is_defined('dimmer'):
 
     class Dimmer(Device):
         """
@@ -24,6 +24,7 @@ if 'dimmer' not in Base.metadata:
         value = Column(Float)
         unit = Column(String)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/distance.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'distance_sensor' not in Base.metadata:
+if not is_defined('distance_sensor'):
 
     class DistanceSensor(NumericSensor):
         """
@@ -18,6 +18,7 @@ if 'distance_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/electricity.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'power_sensor' not in Base.metadata:
+if not is_defined('power_sensor'):
 
     class PowerSensor(NumericSensor):
         __tablename__ = 'power_sensor'
@@ -14,12 +14,13 @@ if 'power_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'current_sensor' not in Base.metadata:
+if not is_defined('current_sensor'):
 
     class CurrentSensor(NumericSensor):
         __tablename__ = 'current_sensor'
@@ -28,12 +29,13 @@ if 'current_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'voltage_sensor' not in Base.metadata:
+if not is_defined('voltage_sensor'):
 
     class VoltageSensor(NumericSensor):
         __tablename__ = 'voltage_sensor'
@@ -42,12 +44,13 @@ if 'voltage_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'energy_sensor' not in Base.metadata:
+if not is_defined('energy_sensor'):
 
     class EnergySensor(NumericSensor):
         __tablename__ = 'energy_sensor'
@@ -56,6 +59,7 @@ if 'energy_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/heart.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'heart_rate_sensor' not in Base.metadata:
+if not is_defined('heart_rate_sensor'):
 
     class HeartRateSensor(NumericSensor):
         """
@@ -18,6 +18,7 @@ if 'heart_rate_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/humidity.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'humidity_sensor' not in Base.metadata:
+if not is_defined('humidity_sensor'):
 
     class HumiditySensor(NumericSensor):
         """
@@ -18,12 +18,13 @@ if 'humidity_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'dew_point_sensor' not in Base.metadata:
+if not is_defined('dew_point_sensor'):
 
     class DewPointSensor(NumericSensor):
         """
@@ -36,6 +37,7 @@ if 'dew_point_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/illuminance.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'illuminance_sensor' not in Base.metadata:
+if not is_defined('illuminance_sensor'):
 
     class IlluminanceSensor(NumericSensor):
         __tablename__ = 'illuminance_sensor'
@@ -14,6 +14,7 @@ if 'illuminance_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/lights.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, String, ForeignKey, Boolean, Float
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .devices import Device
 
 
-if 'light' not in Base.metadata:
+if not is_defined('light'):
 
     class Light(Device):
         __tablename__ = 'light'
@@ -34,6 +34,7 @@ if 'light' not in Base.metadata:
         temperature_min = Column(Float)
         temperature_max = Column(Float)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/linkquality.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'link_quality' not in Base.metadata:
+if not is_defined('link_quality'):
 
     class LinkQuality(NumericSensor):
         __tablename__ = 'link_quality'
@@ -19,6 +19,7 @@ if 'link_quality' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/magnetism.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .three_axis import ThreeAxisSensor
 
 
-if 'magnetometer' not in Base.metadata:
+if not is_defined('magnetometer'):
 
     class Magnetometer(ThreeAxisSensor):
         """
@@ -20,6 +20,7 @@ if 'magnetometer' not in Base.metadata:
             primary_key=True,
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/motion.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import BinarySensor
 
 
-if 'motion_sensor' not in Base.metadata:
+if not is_defined('motion_sensor'):
 
     class MotionSensor(BinarySensor):
         __tablename__ = 'motion_sensor'
@@ -14,6 +14,7 @@ if 'motion_sensor' not in Base.metadata:
             Integer, ForeignKey(BinarySensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/presence.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import BinarySensor
 
 
-if 'presence_sensor' not in Base.metadata:
+if not is_defined('presence_sensor'):
 
     class PresenceSensor(BinarySensor):
         """
@@ -18,6 +18,7 @@ if 'presence_sensor' not in Base.metadata:
             Integer, ForeignKey(BinarySensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/pressure.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'pressure_sensor' not in Base.metadata:
+if not is_defined('pressure_sensor'):
 
     class PressureSensor(NumericSensor):
         """
@@ -18,6 +18,7 @@ if 'pressure_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/sensors.py

@@ -12,7 +12,7 @@ from sqlalchemy import (
     String,
 )
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .devices import Device
 
@@ -32,7 +32,7 @@ class Sensor(Device):
         super().__init__(*args, **kwargs)
 
 
-if 'raw_sensor' not in Base.metadata:
+if not is_defined('raw_sensor'):
 
     class RawSensor(Sensor):
         """
@@ -86,12 +86,13 @@ if 'raw_sensor' not in Base.metadata:
                 self.is_binary = False
                 self.is_json = False
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'numeric_sensor' not in Base.metadata and 'percent_sensor' not in Base.metadata:
+if not is_defined('numeric_sensor') and not is_defined('percent_sensor'):
 
     class NumericSensor(Sensor):
         """
@@ -109,6 +110,7 @@ if 'numeric_sensor' not in Base.metadata and 'percent_sensor' not in Base.metada
         max = Column(Float)
         unit = Column(String)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
@@ -124,6 +126,7 @@ if 'numeric_sensor' not in Base.metadata and 'percent_sensor' not in Base.metada
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
@@ -135,7 +138,7 @@ if 'numeric_sensor' not in Base.metadata and 'percent_sensor' not in Base.metada
             super().__init__(*args, **kwargs)
 
 
-if 'binary_sensor' not in Base.metadata:
+if not is_defined('binary_sensor'):
 
     class BinarySensor(Sensor):
         """
@@ -163,12 +166,13 @@ if 'binary_sensor' not in Base.metadata:
         )
         value = Column(Boolean)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'enum_sensor' not in Base.metadata:
+if not is_defined('enum_sensor'):
 
     class EnumSensor(Sensor):
         """
@@ -184,12 +188,13 @@ if 'enum_sensor' not in Base.metadata:
         values = Column(JSON)
         """ Possible values for the sensor, as a JSON array. """
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'composite_sensor' not in Base.metadata:
+if not is_defined('composite_sensor'):
 
     class CompositeSensor(Sensor):
         """
@@ -204,6 +209,7 @@ if 'composite_sensor' not in Base.metadata:
         )
         value = Column(JSON)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/steps.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'steps_sensor' not in Base.metadata:
+if not is_defined('steps_sensor'):
 
     class StepsSensor(NumericSensor):
         """
@@ -18,6 +18,7 @@ if 'steps_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/switches.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey, Boolean, String, JSON
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .devices import Device
 
 
-if 'switch' not in Base.metadata:
+if not is_defined('switch'):
 
     class Switch(Device):
         __tablename__ = 'switch'
@@ -15,12 +15,13 @@ if 'switch' not in Base.metadata:
         )
         state = Column(Boolean)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'enum_switch' not in Base.metadata:
+if not is_defined('enum_switch'):
 
     class EnumSwitch(Device):
         __tablename__ = 'enum_switch'
@@ -31,6 +32,7 @@ if 'enum_switch' not in Base.metadata:
         value = Column(String)
         values = Column(JSON)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/system.py

@@ -1,6 +1,6 @@
 from sqlalchemy import Boolean, Column, Float, ForeignKey, Integer, JSON, String
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from . import Entity
 from .devices import Device
@@ -8,7 +8,7 @@ from .sensors import NumericSensor, PercentSensor
 from .temperature import TemperatureSensor
 
 
-if 'cpu' not in Base.metadata:
+if not is_defined('cpu'):
 
     class Cpu(Entity):
         """
@@ -23,12 +23,13 @@ if 'cpu' not in Base.metadata:
 
         percent = Column(Float)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'cpu_info' not in Base.metadata:
+if not is_defined('cpu_info'):
 
     class CpuInfo(Entity):
         """
@@ -54,12 +55,13 @@ if 'cpu_info' not in Base.metadata:
         l2_cache_size = Column(Integer)
         l3_cache_size = Column(Integer)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'cpu_times' not in Base.metadata:
+if not is_defined('cpu_times'):
 
     class CpuTimes(Entity):
         """
@@ -72,12 +74,13 @@ if 'cpu_times' not in Base.metadata:
             Integer, ForeignKey(Entity.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'cpu_stats' not in Base.metadata:
+if not is_defined('cpu_stats'):
 
     class CpuStats(Entity):
         """
@@ -90,12 +93,13 @@ if 'cpu_stats' not in Base.metadata:
             Integer, ForeignKey(Entity.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'memory_stats' not in Base.metadata:
+if not is_defined('memory_stats'):
 
     class MemoryStats(Entity):
         """
@@ -119,12 +123,13 @@ if 'memory_stats' not in Base.metadata:
         shared = Column(Integer)
         percent = Column(Float)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'swap_stats' not in Base.metadata:
+if not is_defined('swap_stats'):
 
     class SwapStats(Entity):
         """
@@ -142,12 +147,13 @@ if 'swap_stats' not in Base.metadata:
         free = Column(Integer)
         percent = Column(Float)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'disk' not in Base.metadata:
+if not is_defined('disk'):
 
     class Disk(Entity):
         """
@@ -175,12 +181,13 @@ if 'disk' not in Base.metadata:
         write_time = Column(Float)
         busy_time = Column(Float)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'network_interface' not in Base.metadata:
+if not is_defined('network_interface'):
 
     class NetworkInterface(Device):
         """
@@ -207,12 +214,13 @@ if 'network_interface' not in Base.metadata:
         duplex = Column(String)
         flags = Column(JSON)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'system_temperature' not in Base.metadata:
+if not is_defined('system_temperature'):
 
     class SystemTemperature(TemperatureSensor):
         """
@@ -230,12 +238,13 @@ if 'system_temperature' not in Base.metadata:
         high = Column(Float)
         critical = Column(Float)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'system_fan' not in Base.metadata:
+if not is_defined('system_fan'):
 
     class SystemFan(NumericSensor):
         """
@@ -250,12 +259,13 @@ if 'system_fan' not in Base.metadata:
             primary_key=True,
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }
 
 
-if 'system_battery' not in Base.metadata:
+if not is_defined('system_battery'):
 
     class SystemBattery(PercentSensor):
         """
@@ -273,6 +283,7 @@ if 'system_battery' not in Base.metadata:
         seconds_left = Column(Float)
         power_plugged = Column(Boolean)
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/temperature.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'temperature_sensor' not in Base.metadata:
+if not is_defined('temperature_sensor'):
 
     class TemperatureSensor(NumericSensor):
         __tablename__ = 'temperature_sensor'
@@ -14,6 +14,7 @@ if 'temperature_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/three_axis.py

@@ -1,13 +1,13 @@
 from typing import Iterable, Mapping, Optional, Union
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 from platypush.common.sensors import Numeric
 
 from .sensors import RawSensor
 
 
-if 'three_axis_sensor' not in Base.metadata:
+if not is_defined('three_axis_sensor'):
 
     class ThreeAxisSensor(RawSensor):
         """
@@ -20,6 +20,7 @@ if 'three_axis_sensor' not in Base.metadata:
             Integer, ForeignKey(RawSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/time.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'time_duration' not in Base.metadata:
+if not is_defined('time_duration'):
 
     class TimeDuration(NumericSensor):
         """
@@ -18,6 +18,7 @@ if 'time_duration' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }

platypush/entities/variables.py

@@ -2,14 +2,14 @@ import logging
 
 from sqlalchemy import Column, ForeignKey, Integer, String
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from . import Entity
 
 logger = logging.getLogger(__name__)
 
 
-if 'variable' not in Base.metadata:
+if not is_defined('variable'):
 
     class Variable(Entity):
         """

platypush/entities/weight.py

@@ -1,11 +1,11 @@
 from sqlalchemy import Column, Integer, ForeignKey
 
-from platypush.common.db import Base
+from platypush.common.db import is_defined
 
 from .sensors import NumericSensor
 
 
-if 'weight_sensor' not in Base.metadata:
+if not is_defined('weight_sensor'):
 
     class WeightSensor(NumericSensor):
         """
@@ -18,6 +18,7 @@ if 'weight_sensor' not in Base.metadata:
             Integer, ForeignKey(NumericSensor.id, ondelete='CASCADE'), primary_key=True
         )
 
+        __table_args__ = {'extend_existing': True}
         __mapper_args__ = {
             'polymorphic_identity': __tablename__,
         }