- A Simple Example
- Detecting Changes
- A Complete Example
- Conclusion
- ZODB Resources
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A Complete Example
The following is a complete example program. It builds on the employee examples used so far.
Listing 1 A Complete Example Program
from ZODB import DB
from ZODB.FileStorage import FileStorage
from ZODB.PersistentMapping import PersistentMapping
from Persistence import Persistent
class Employee(Persistent):
"""An employee"""
def __init__(self, name, manager=None):
self.name=name
self.manager=manager
# setup the database
storage=FileStorage("employees.fs")
db=DB(storage)
connection=db.open()
root=connection.root()
# get the employees mapping, creating an empty mapping if
# necessary
if not root.has_key("employees"):
root["employees"] = {}
employees=root["employees"]
def listEmployees():
if len(employees.values())==0:
print "There are no employees."
print
return
for employee in employees.values():
print "Name: %s" % employee.name
if employee.manager is not None:
print "Manager's name: %s" %
employee.manager.name
print
def addEmployee(name, manager_name=None):
if employees.has_key(name):
print "There is already an employee with this
name."
return
if manager_name:
try:
manager=employees[manager_name]
except KeyError:
print
print "No such manager"
print
return
employees[name]=Employee(name, manager)
else:
employees[name]=Employee(name)
root['employees'] = employees # reassign to
change
get_transaction().commit()
print "Employee %s added." % name
print
if __name__=="__main__":
while 1:
choice=raw_input("Press 'L' to list employees,
'A' to"
"add an employee, or 'Q' to
quit:")
choice=choice.lower()
if choice=="l":
listEmployees()
elif choice=="a":
name=raw_input("Employee name:")
manager_name=raw_input("Manager name:")
addEmployee(name, manager_name)
elif choice=="q":
break
# close database
connection.close()
This program demonstrates a couple of interesting things. First, this program shows how persistent objects can refer to each other. The self.manager attribute of employee instances can refer to other employee instances. Unlike a relational database, there is no need to use indirection, such as object ids, when referring from one persistent object to another. You can just use normal Python references. In fact, you can even use circular references.
A final trick used by this program is to look for a persistent object and create it if it is not present. This allows you to just run this program without having to run a setup script to build the database first. If there is no database present, the program will create one and initialize it.