Is possible to create Column in SQLAlchemy which is going to be automatically populated with time when it inserted/updated last time?
Question:
Is it possible to create Column in SQLAlchemy which is going to be automatically populated with time when it inserted/updated last time ?
I created models, inherited from Base class
class Base(object):
def __tablename__(self):
return self.__name__.lower()
id = Column(Integer, primary_key=True)
last_time = Column(TIMESTAMP, server_default=func.now())
Base = declarative_base(cls=Base)
class EntityModel(Base):
__tablename__ = 'entities'
settlement_id = Column(Integer, ForeignKey('settlements.id'), nullable=False)
type = Column(String(20), nullable=False)
level = Column(Integer, nullable=False, default=0)
energy = Column(Float, nullable=False, default=0)
position_x = Column(Integer, default=0)
position_y = Column(Integer, default=0)
def __repr__(self):
return "<Entity('%s')>" % (self.type)
Every time when I update EntityModel I want to the last_time
be updated on system function.now()
. I can do this on the database level with triggers but I would rather do on application level if is it possible.
Answers:
In Base class add onupdate in the last statement as follows:
from sqlalchemy.sql import func
last_time = Column(TIMESTAMP, server_default=func.now(), onupdate=func.current_timestamp())
If you use MySQL, I believe you can only have one auto-updating datetime column, so we use SQLAlchemy’s event triggers instead.
You just attach a listener to the ‘before_insert’ and ‘before_update’ hooks and update as necessary:
from sqlalchemy import event
@event.listen(YourModel, 'before_insert')
def update_created_modified_on_create_listener(mapper, connection, target):
""" Event listener that runs before a record is updated, and sets the create/modified field accordingly."""
target.created = datetime.utcnow()
target.modified = datetime.utcnow()
@event.listen(YourModel, 'before_update')
def update_modified_on_update_listener(mapper, connection, target):
""" Event listener that runs before a record is updated, and sets the modified field accordingly."""
# it's okay if this field doesn't exist - SQLAlchemy will silently ignore it.
target.modified = datetime.utcnow()
I knew nobody would ever remember to add this to new models, so I tried to be clever and add it for them.
All our models inherit from a base object we cleverly called “DatabaseModel”. We check who inherits from this object and dynamically add the triggers to all of them.
It’s OK if a model doesn’t have the created or modified field – SQLAlchemy appears to silently ignore it.
class DatabaseModel(db.Model):
__abstract__ = True
#...other stuff...
@classmethod
def _all_subclasses(cls):
""" Get all subclasses of cls, descending. So, if A is a subclass of B is a subclass of cls, this
will include A and B.
(Does not include cls) """
children = cls.__subclasses__()
result = []
while children:
next = children.pop()
subclasses = next.__subclasses__()
result.append(next)
for subclass in subclasses:
children.append(subclass)
return result
def update_created_modified_on_create_listener(mapper, connection, target):
""" Event listener that runs before a record is updated, and sets the create/modified field accordingly."""
# it's okay if one of these fields doesn't exist - SQLAlchemy will silently ignore it.
target.created = datetime.utcnow()
target.modified = datetime.utcnow()
def update_modified_on_update_listener(mapper, connection, target):
""" Event listener that runs before a record is updated, and sets the modified field accordingly."""
# it's okay if this field doesn't exist - SQLAlchemy will silently ignore it.
target.modified = datetime.utcnow()
for cls in DatabaseModel._all_subclasses():
event.listen(cls, 'before_insert', update_created_modified_on_create_listener)
event.listen(cls, 'before_update', update_modified_on_update_listener)
It is worth nothing that, if you follow Rachel Sanders’ recommendation, you should definitely do:
if object_session(target).is_modified(target, include_collections=False):
target.modified = datetime.utcnow()
as part of the update_modified_on_update_listener() event listener, otherwise you’ll do tons of redundant database updates. Checkout http://docs.sqlalchemy.org/en/latest/orm/events.html#mapper-events under the section “before_update” for more information.
Is it possible to create Column in SQLAlchemy which is going to be automatically populated with time when it inserted/updated last time ?
I created models, inherited from Base class
class Base(object):
def __tablename__(self):
return self.__name__.lower()
id = Column(Integer, primary_key=True)
last_time = Column(TIMESTAMP, server_default=func.now())
Base = declarative_base(cls=Base)
class EntityModel(Base):
__tablename__ = 'entities'
settlement_id = Column(Integer, ForeignKey('settlements.id'), nullable=False)
type = Column(String(20), nullable=False)
level = Column(Integer, nullable=False, default=0)
energy = Column(Float, nullable=False, default=0)
position_x = Column(Integer, default=0)
position_y = Column(Integer, default=0)
def __repr__(self):
return "<Entity('%s')>" % (self.type)
Every time when I update EntityModel I want to the last_time
be updated on system function.now()
. I can do this on the database level with triggers but I would rather do on application level if is it possible.
In Base class add onupdate in the last statement as follows:
from sqlalchemy.sql import func
last_time = Column(TIMESTAMP, server_default=func.now(), onupdate=func.current_timestamp())
If you use MySQL, I believe you can only have one auto-updating datetime column, so we use SQLAlchemy’s event triggers instead.
You just attach a listener to the ‘before_insert’ and ‘before_update’ hooks and update as necessary:
from sqlalchemy import event
@event.listen(YourModel, 'before_insert')
def update_created_modified_on_create_listener(mapper, connection, target):
""" Event listener that runs before a record is updated, and sets the create/modified field accordingly."""
target.created = datetime.utcnow()
target.modified = datetime.utcnow()
@event.listen(YourModel, 'before_update')
def update_modified_on_update_listener(mapper, connection, target):
""" Event listener that runs before a record is updated, and sets the modified field accordingly."""
# it's okay if this field doesn't exist - SQLAlchemy will silently ignore it.
target.modified = datetime.utcnow()
I knew nobody would ever remember to add this to new models, so I tried to be clever and add it for them.
All our models inherit from a base object we cleverly called “DatabaseModel”. We check who inherits from this object and dynamically add the triggers to all of them.
It’s OK if a model doesn’t have the created or modified field – SQLAlchemy appears to silently ignore it.
class DatabaseModel(db.Model):
__abstract__ = True
#...other stuff...
@classmethod
def _all_subclasses(cls):
""" Get all subclasses of cls, descending. So, if A is a subclass of B is a subclass of cls, this
will include A and B.
(Does not include cls) """
children = cls.__subclasses__()
result = []
while children:
next = children.pop()
subclasses = next.__subclasses__()
result.append(next)
for subclass in subclasses:
children.append(subclass)
return result
def update_created_modified_on_create_listener(mapper, connection, target):
""" Event listener that runs before a record is updated, and sets the create/modified field accordingly."""
# it's okay if one of these fields doesn't exist - SQLAlchemy will silently ignore it.
target.created = datetime.utcnow()
target.modified = datetime.utcnow()
def update_modified_on_update_listener(mapper, connection, target):
""" Event listener that runs before a record is updated, and sets the modified field accordingly."""
# it's okay if this field doesn't exist - SQLAlchemy will silently ignore it.
target.modified = datetime.utcnow()
for cls in DatabaseModel._all_subclasses():
event.listen(cls, 'before_insert', update_created_modified_on_create_listener)
event.listen(cls, 'before_update', update_modified_on_update_listener)
It is worth nothing that, if you follow Rachel Sanders’ recommendation, you should definitely do:
if object_session(target).is_modified(target, include_collections=False):
target.modified = datetime.utcnow()
as part of the update_modified_on_update_listener() event listener, otherwise you’ll do tons of redundant database updates. Checkout http://docs.sqlalchemy.org/en/latest/orm/events.html#mapper-events under the section “before_update” for more information.