from sqlalchemy.orm import subqueryload, joinedload, eagerload
from sqlalchemy import Column, DateTime, String, Integer, ForeignKey, func,Float, sql
from sqlalchemy.orm import relation
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from sqlalchemy import create_engine
engine = create_engine('sqlite:///testdb.sqlite', echo=True)
session = sessionmaker()
session.configure(bind=engine)
Base = declarative_base()
Base.metadata.create_all(engine)
s= session()
class Stock(Base):
__tablename__ = 'stock'
stock_id = Column(Integer, primary_key=True)
name = Column(String)
prices = relation("StockPrice")
class StockPrice(Base):
__tablename__ = 'stock_price'
stock_id = Column(Integer, ForeignKey('stock.stock_id'), primary_key=True)
date = Column(String, primary_key=True)
price = Column(Float)
source = Column(String, primary_key=True)
user = Column(String)
Base.metadata.create_all(engine)
stockprice1 = StockPrice(stock_id = 1, date="2014-10-29", price="170.0", source="X Firm", user="U1")
stockprice2 = StockPrice(stock_id = 1, date="2014-10-30", price="175.0", source="X Firm", user="U2")
stock1 = Stock(stock_id = 1, name = "GOOGLE", prices=[stockprice1, stockprice2])
stockprice1 = StockPrice(stock_id = 2, date="2014-10-29", price="150.0", source="X Firm", user="U1")
stockprice2 = StockPrice(stock_id = 2, date="2014-10-30", price="155.0", source="X Firm", user="U2")
stock2 = Stock(stock_id = 2, name = "YAHOO", prices=[stockprice1, stockprice2])
s.add_all([stock1, stock2])
s.commit()
Eager loading of the price for a stock:
stock = s.query(Stock).options(joinedload(Stock.prices)).filter(Stock.stock_id == 1).one()
One way to eagerly load price for a stock for a given date:
stock = s.query(Stock).options(joinedload(Stock.prices)).filter(Stock.stock_id == 1).filter(StockPrice.date == "2014-10-30").one()
But the problem with the approach is if you have may tables like StockPrice related to Stock and if you want to load the all relations for a given date then the result set becomes very huge after joining all relations. Filter adds the conditions in WHERE clause, instead I need a way to specify condition on join to eagerly load.
stock = s.query(Stock).options(joinedload(Stock.prices, #condition does not work here)).filter(Stock.stock_id == 1).one()
Instead of joinedload(Stock.prices)
do the following:
stock = (s.query(Stock)
# @note: this replaces `joinedload(Stock.prices)`
.join(StockPrice,
and_(StockPrice.stock_id == Stock.stock_id,
StockPrice.date == "2014-10-30")
)
# effectively *trick* SQLAlchemy into thinking that above we loaded all
# items for the relationship *Stock.prices*
.options(contains_eager(Stock.prices))
).get(1) # will retrieve the instance for Stock.stock_id = 1