Using Models

Defining Models

Models are defined by extending the db.Model class and are comprised of many Datatypes. It is recommended to use the primary_key decorator to define the model’s primary key.

from cerami.datatype import String, Number
from cerami.decorators import primary_key

@primary_key('name')
class Person(db.Model):
    __tablename__ = "People"

    name = String()
    age = Number()
    favorite_drink = String()

    def say_hello(self):
        print(f'Hello {self.name}!')

Instantiating Models

person = Person(name="Bobby Boucher", age=26, favorite_drink="water")
person.say_hello()

Saving Records (Put / Update)

DynamoDB has two different save operations. put_item and update_item.

• Put will replace the entire item (or create it if its missing)

• Update works like a patch and only overwrites fields that are part of the update

bobby = Person(name="Bobby Boucher", age=26, favorite_drink="water")
bobby.put() # creates Bobby

Cerami always creates records based off of the full column set. If an attribute is missing from the object, it will be saved to the database as a null object using DynamoDB’s syntax {NULL: True}

billy = Person(name="Billy Madison", age=27)
# favorite_drink will be saved as None in the database
billy.put()

Cerami keeps track of which columns have changed and will only update these different values.

billy_again = Person(name="Billy Madison", favorite_drink="beer")
# Even though age is initialized as None, it will not be saved in the database as such
billy_again.update()

billy_from_db = Person.get.key(name="Billy Madison").execute().item
billy_from_db.as_item()
{'name': {'S': 'Billy Madison'}, 'age': {'N': '27'}, 'favorite_drink': {'S': 'beer'}}

Deleting Models

billy.delete()

Viewing Model Data

DynamoDB expects its data formatted in a specific dict format. This can be done automatically with the as_item() method.

bobby = Person(name="Bobby Boucher", age=26, favorite_drink="water")

bobby.as_item()
{'name': {'S': 'Bobby Boucher'}, 'age': {'N': '26'}, 'favorite_drink': {'S': 'water'}}