7. Polymorphism and Inheritance

This part of the documentation provides an overview of implementing and using polymorphism and inheritance using the motormongo framework, enabling flexible and organized data models for various use cases.

7.1. Base Model: Item

The Item class serves as the base model for different types of items stored in a MongoDB collection. It defines common fields and methods that are shared across all item types.

from motormongo import Document, StringField, FloatField


class Item(Document):
    name = StringField()
    cost = FloatField()

7.2. Subclass Models

Subclasses of Item can introduce specific fields or override methods to cater to different item categories.

7.2.1. Book

A Book represents a specific type of Item with additional attributes related to books.

class Book(Item):
    title = StringField()
    author = StringField()
    isbn = StringField()

7.2.2. Electronics

An Electronics item represents electronic goods with attributes like warranty period and brand.

class Electronics(Item):
    warranty_period = StringField()  # E.g., "2 years"
    brand = StringField()

7.3. Usage

7.3.1. Creating and Inserting Items

To insert items into the database, use the insert_one method. The item’s type is managed automatically.

# Insert a book
book = await Book.insert_one(title="1984", author="George Orwell", isbn="123456789", cost=20.0, name="Book")

# Insert an electronics item
electronics = await Electronics.insert_one(warranty_period="2 years", brand="TechBrand", cost=999.99, name="Laptop")

7.3.2. Querying Items

You can query items of any type using their base or specific models. Polymorphism allows retrieved instances to be of the correct subclass.

# Find a book by ISBN
found_book = await Book.find_one(isbn="123456789")

# Find an electronics item by brand
found_electronics = await Electronics.find_one(brand="TechBrand")

7.4. Polymorphic Behavior

The following operations are supported over the base Item Document class, enabling complex querying over base Item Document class and all of its subclasses (i.e Book and Electronics):

CRUD Type	Operation
Read	`find_many(query: dict, limit: int = None, return_as_list: bool = True **kwargs) -> List[Document]`
Update	`update_many(query: dict, update_fields: dict) -> Tuple[List[Document], int]`
Delete	`delete_many(query: dict, **kwargs) -> int`

As well as aggregate operations, see the Aggregation Operation section for more details.

Querying on the base Item model returns items of all types, correctly instantiated as their specific subclasses. See below for a logical example of polymorphic querying:

# Find all items with a cost over 50
expensive_items = await Item.find_many(cost={"$gt": 50})

for item in expensive_items:
    print(type(item))  # Prints the subclass (Book, Electronics, etc.)
    if isinstance(item, Book):
        print(f"Book: {item.title} by {item.author}")
    elif isinstance(item, Electronics):
        print(f"Electronics: {item.brand} with {item.warranty_period} warranty")