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Insert column based on related column value

This seems like a real beginner question, but I'm having trouble finding a simple answer. I have simplified this down to just the bare bones with a simple data model representing a one-to-many relationship:
class Room(db.Model):
__tablename__ = 'rooms'
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(128), unique=True)
capacity = db.Column(db.Integer)
events = db.relationship('Event', backref='room')
class Event(db.Model):
__tablename__ = 'counts'
id = db.Column(db.Integer, primary_key=True)
unusedCapacity = db.Column(db.Integer)
attendance = db.Column(db.Integer)
room_id = db.Column(db.Integer, db.ForeignKey('rooms.id'))
Event.unusedCapacity is calculated as Room.capacity - Event.attendance, but I need to store the value in the column — Room.capacity may change over time, but the Event.unusedCapacity needs to reflect the actual unused capacity at the time of the Event.
I am currently querying the Room and then creating the event:
room = Room.query.get(room_id) # using Flask sqlAlchemy
event = event(unusedCapacity = room.capacity - attendance, ...etc)
My question is: is there a more efficient way to do this in one step?

As noted in the comments by #SuperShoot, a query on insert can calculate the unused capacity in the database without having to fetch first. An explicit constructor, such as shown by #tooTired, could pass a scalar subquery as unusedCapacity:
class Event(db.Model):
...
def __init__(self, **kwgs):
if 'unusedCapacity' not in kwgs:
kwgs['unusedCapacity'] = \
db.select([Room.capacity - kwgs['attendance']]).\
where(Room.id == kwgs['room_id']).\
as_scalar()
super().__init__(**kwgs)
Though it is possible to use client-invoked SQL expressions as defaults, I'm not sure how one could refer to the values to be inserted in the expression without using a context-sensitive default function, but that did not quite work out: the scalar subquery was not inlined and SQLAlchemy tried to pass it using placeholders instead.
A downside of the __init__ approach is that you cannot perform bulk inserts that would handle unused capacity using the table created for the model as is, but will have to perform a manual query that does the same.
Another thing to look out for is that until a flush takes place the unusedCapacity attribute of a new Event object holds the SQL expression object, not the actual value. The solution by #tooTired is more transparent in this regard, since a new Event object will hold the numeric value of unused capacity from the get go.

SQLAlchemy adds an implicit constructor to all model classes which accepts keyword arguments for all its columns and relationships. You can override this and pass the kwargs without unusedCapacity and get the room capacity in the constructor:
class Event(db.Model):
# ...
#kwargs without unusedCapacity
def __init__(**kwargs):
room = Room.query.get(kwargs.get(room_id))
super(Event, self).__init__(unusedCapacity = room.capacity - kwargs.get(attendance), **kwargs)
#Create new event normally
event = Event(id = 1, attendance = 1, room_id = 1)

SQLAlchemy Replicate Client Side Method on SQL-Side

Note: This is a simplified example of what I'm actually trying to do here.
I have the following Parent-Child relationship both driven off a declarative_base.
class Parent(declartive_base):
__tablename__ = 'parents'
id = Column(Integer, primary_key=True)
_children = relationship("Child", lazy='dynamic')
def total_for_date(self, date):
return sum([child.num for child in self._children.filter(Child.date == date)])
#classmethod
def total_for_date_query(cls, date):
#TODO Return a query that represents this...
pass
class Child(declarative_base):
__tablename__ = 'children'
id = Column(Integer, primary_key=True)
num = Column(Integer)
date = Column(Date)
parent_id = Column(Integer, ForeignKey('parents.id'))
_parent = relationship("Parent")
I'd like to calculate a total of a certain number associated with a child given a parent query. This can be performed via python as such
q = session.query(Parent).filter(Parent.id_([4,5,10,...]))
total = sum([parent.total_for_date(datetime.date(2018, 1, 2)) for parent in q.all()])
However, the computation here is done in python and given a large amount of data, won't perform as well compared to SQL.
I'm trying to figure out a way using hybrid expressions, selects, sqlalchemy queries etc. to have an equivalent method on the parent that returns a query/selectable/expression that will allow me to perform the computation on the SQL side, but maintain a similar interface compared to the other method.
In this example, I'd would like to do the following instead.
q = session.query(Parent).filter(Parent.id.in_([4,5,10]))
total = q.select_entity_from(Parent.total_for_date_query(datetime.date(2018, 1, 2))).scalar()
#Note idk if "select_entity_from" is what I want here
But I don't know how to fill out the SQL-side method equivalent total_for_date_query. I just can't seem to wrap my head around when to use a Query vs. Selectable, hybrid property expressions vs. hybrid method expressions etc.

SQLAlchemy update parent when related child changes

I'm trying to model an entity that as one or more one-to-many relationships, such that it's last_modified attribute is updated, when
a child is added or removed
a child is modified
the entity itself is modified
I've put together the following minimal example:
class Config(Base):
__tablename__ = 'config'
ID = Column('ID', Integer, primary_key=True)
name = Column('name', String)
last_modified = Column('last_modified', DateTime, default=now, onupdate=now)
params = relationship('ConfigParam', backref='config')
class ConfigParam(Base):
__tablename__ = 'config_params'
ID = Column('ID', Integer, primary_key=True)
ConfigID = Column('ConfigID', Integer, ForeignKey('config.ID'), nullable=False)
key = Column('key', String)
value = Column('value', Float)
#event.listens_for(Config.params, 'append')
#event.listens_for(Config.params, 'remove')
def receive_append_or_remove(target, value, initiator):
target.last_modified = now()
#event.listens_for(ConfigParam.key, 'set')
#event.listens_for(ConfigParam.value, 'set')
def receive_attr_change(target, value, oldvalue, initiator):
if target.config:
# don't act if the parent config isn't yet set
# i.e. during __init__
target.config.last_modified = now()
This seems to work, but I'm wondering if there's a better way to do this?
Specifically, this becomes very verbose since my actual ConfigParam implementation has more attributes and I'm having multiple one-to-many relations configured on the parent Config class.

Take this with a huge grain of salt, it "seems" to work, could explode:
def rel_listener(t, v, i):
t.last_modified = now()
def listener(t, v, o, i):
if t.config:
t.config.last_modified = now()
from sqlalchemy import inspect
for rel in inspect(Config).relationships:
event.listen(rel, 'append', rel_listener)
event.listen(rel, 'remove', rel_listener)
for col in inspect(ConfigParam).column_attrs:
event.listen(col, 'set', listener)
Problem is that the inspections make no exceptions and columns such as 'ID' and 'ConfigID' will be bound to event listeners.
Another perhaps slightly less tedious form would be to just use a list of attributes to bind events to in a similar fashion:
for attr in ['key', 'value']:
event.listen(getattr(ConfigParam, attr), 'set', listener)
This gives you control over what is bound to events and what is not.

sqlalchemy add child in one-to-many relationship

This is the first time I've used ORM, so I'm not sure the best way to handle this. I have a one-to-many relationship where each Parent can have many Children:
class Parent(Base):
__tablename__ = 'Parent'
name = Column(String(50))
gid = Column(String(16), primary_key = True)
lastUpdate = Column(DateTime)
def __init__(self,name, gid):
self.name = name
self.gid = gid
self.lastUpdate = datetime.datetime.now()
class Child(Base):
__tablename__ = 'Child'
id = Column(Integer, primary_key = True)
loc = Column(String(50))
status = Column(String(50))
parent_gid = Column(String(16), ForeignKey('Parent.gid'))
parent = relationship("Parent", backref=backref('children'))
Now, updates are coming in over the network. When an update comes in, I want to UPDATE the appropriate Parent row (updating lastUpdate column) and INSERT new children rows into the database. I don't know how to do that with ORM. Here is my failed attempt:
engine = create_engine('sqlite+pysqlite:///file.db',
module=dbapi2)
Base.metadata.create_all(engine)
session = sessionmaker(bind=engine)()
def addChildren(parent):
p = session.query(Parent).filter(Parent.gid == p1.gid).all()
if len(p) == 0:
session.add(p1)
session.commit()
else:
updateChildren = parent.children[:]
parent.chlidren = []
for c in updateChildren:
c.parent_gid = parent.gid
session.add_all(updateChildren)
session.commit()
if __name__ == '__main__':
#first update from the 'network'
p1 = Parent(name='team1', gid='t1')
p1.children = [Child(loc='x', status='a'), Child(loc='y', status='b')]
addChildren(p1)
import time
time.sleep(1)
#here comes another network update
p1 = Parent(name='team1', gid='t1')
p1.children = [Child(loc='z', status='a'), Child(loc='k', status='b')]
#this fails
addChildren(p1)
I initially tried to do a merge, but that caused the old children to be disassociated with the parent (the foreign IDs were set to null). What is the best way to approach this with ORM? Thanks
EDIT
I guess it doesn't really make sense to create entirely new objects when updates come in over the network. I should just query the session for the appropriate parent, then create new children if necessary and merge? E.g.
def addChildren(pname, pid, cloc, cstat):
p = session.query(Parent).filter(Parent.gid == pid).all()
if len(p) == 0:
p = Parent(pname, pid)
p.children = [Child(loc=cloc, status=cstat)]
session.add(p)
session.commit()
else:
p = p[0]
p.children.append(Child(loc=cloc, status=cstat))
session.merge(p)
session.commit()

You are right - you should not create the same parent twice. In terms of adding children, ... well, you really need only to add them and you do not care about the existing ones... So your edited code should do the job just fine. You can make it shorter and more readable though:
def addChildren(pname, pid, cloc, cstat):
p = session.query(Parent).get(pid) # will give you either Parent or None
if not(p):
p = Parent(pname, pid)
session.add(p)
p.children.append(Child(loc=cloc, status=cstat))
session.commit()
The disadvantage of this way is that for existing Parent the whole collection of Children will be loaded into memory before a new Child is added and later saved to the database. If this is the case (many and increasing number of children for each parent), then the lazy='noload' might be useful:
parent = relationship("Parent", backref=backref('children', lazy='noload'))
This might dramatically improve the speed of inserts, but in this case the access to p.children will never load the existing objects from the database. In such scenarios it is enough to define another relationship. In these situations I prefer to use Building Query-Enabled Properties, so you end up with one property only for adding objects, and the other only for quering persisted results, which often are used by different parts of the system.

SQLAlchemy inheritance based on whether data exists in another table

I have two legacy table that I would like to use SQLAlchemy declarative to access data.
Order:
order_id
is_processed
FooData:
foo_id
order_id
A order may or may not have FooData and I would like to distinguish between the two order types using SQLAlchemy declarative models.
The problem I have wrapping my head around is.
How do I set up such a relationship? Ideally I'd have two classes Order and FooOrder where Order has no FooData and FooOrder has FooData.
I have to query both types (Order and FooOrder) together based on is_processed and process them differently based on whether it is Order or FooOrder. How do I go about querying in this case?

If you can change the DB, then simply add one discriminator column, set
the value of this column to proper value (order|foodata) depending on whether
the foodata exists for it, make it NOT NULL and configure simple Joined Table Inheritance.
If you cannot change the DB (add a discriminator column) and you only have the simple
2-table model as you show, then I would not use inheritance, but rather 1-1 relationship.
Model Definition:
class Order(Base):
__tablename__ = 'order'
__table_args__ = {'autoload': True}
class FooData(Base):
__tablename__ = 'foo_data'
__table_args__ = {'autoload': True}
# #note: you need next line only if your DB does not have FK defined
#__table_args__ = (ForeignKeyConstraint(['order_id'], ['order.order_id']), {'autoload': True})
# define 1-[0..1] relationship from Order to FooData with eager loading
Order.foodata = relationship(FooData, uselist=False, lazy="joined", backref="order")
Adding new objects:
ord = Order(); ord.is_processed = False
session.add(ord)
ord = Order(); ord.is_processed = False
foo = FooData(); foo.someinfo = "test foo created from SA"
ord.foodata = foo
session.add(ord)
session.commit()
Query: all based on is_processed:
qry = session.query(Order).filter(Order.is_processed == False)
for ord in qry:
print ord, ord.foodata
A la Polymorphism:
You can even implement methods on your Order and FooData in a way that it
would seem they are in fact using inheritance:
class Order(Base):
# ...
def process_order(self):
if self.foodata:
self.foodata.process_order()
else:
print "processing order: ", self
class FooData(Base):
# ...
def process_order(self):
print "processing foo_data: ", self