Seeding

Django Seeding Best Practices

Common Mistakes

Model Choice Fields

• On django char fields that are linked to choices user faker.
• It will automatically pull from our selected choices.

    fields = {
        'type': 'faker'
    }

Overuser of faker

• Faker should be used to generate numerical data.
• Faker should be used for dates and type fields.
• Majority of the time we want to give the LLM context to generate us the best data.

Foreign Key Relationships

• Field names must be appended with _id
• Only choose from the custom fk_random or fk_in_order methods when seeding
• The related objects must be imported.
• You cannot use a string for the model class import. It must be the direct import

fields: ClassVar = {    
        'product_id': ('custom', 'fk_in_order', {'model_class': Product}),        
        'inventory_id': ('custom', 'fk_random', {'model_class': Invenotry}),        
    }

Seeding Datetimes

• When seeding datetimes we need to use the custom date_time_between. This follow the same syntax as faker.
• Faker does not make the date times timezone aware. This is why we need to use this.

Root seed.py import order

• It is very important to order our seed imports in a logical order.
• Modules depend on one another. Attempting to seed a dependent model before its relationship will result in data loss.

Choose Appropriate Data Generation Strategies

Different data generation strategies serve different purposes:

LLM (Large Language Models)

• Best for rich, human-like text content
• Automatically generates descriptive content for fields like descriptions
• Example: ("llm", "Describe this product for a sales catalog.")

Faker

• Ideal for realistic structured data (names, dates, numbers)
• Generates consistent, believable data for fields like names, addresses, and dates
• Example: ("faker", "word") or ("faker", "date_time_between", {"start_date": "-30d", "end_date": "now"})

Static

• Use for consistent values like feature flags or test conditions
• Provides the same value every time for predictable results
• Example: ("static", True) or "in_stock": True

Callable

• For dynamic behavior like timestamps or context-aware generation
• Executes a function at runtime to generate values
• Example: ("callable", lambda: timezone.now())

Custom

• For reusable methods like sequential foreign key assignment
• Critical for maintaining referential integrity in foreign key relationships
• Example: ("custom", "in_order", {"values": supplier_ids})
• Important: When using custom methods for foreign keys, ensure proper caching to maintain data consistency

Configure Default Behavior Appropriately

class ProductSeeder(DjangoModelSeeder):
    model_class = Product
    default_to = "llm"  # Options: 'llm', 'faker'

Field Configuration Patterns

Basic Field Definitions

fields = {
    'id': 'exclude',
    'name': ('llm', 'A product in a clothing sales catalog'),
    'description': ('llm', 'Describe this product for a clothing sales catalog.'),
    'price': ('faker', 'pydecimal', {'left_digits': 2, 'right_digits': 2, 'positive': True}),
    'in_stock': True,
    'created_at': ('faker', 'date_time_between', {'start_date': '-30d', 'end_date': 'now'}),
    'updated_at': lambda: timezone.now(),
    'supplier_id': ('custom', 'in_order', {'values': supplier_ids})
}

Advanced Field Types

• Faker: Use for quick realistic structured data
• LLM: Generate rich text content. Used in most cases.
• Static: Fixed values for consistency
• Callable: Dynamic runtime evaluation
• Custom: Reusable methods for complex logic, especially foreign key relationships

Implementation Guidelines

1. Cache Configuration

Always configure caching for frequently-used seeders:

class ProductSeeder(DjangoModelSeeder):
    cache_name = 'product_seeder'
    cache_seed = True

2. Field Selection Strategy

• Use default_to = "llm" or "faker" for automatic field population
• Explicitly define fields that should be excluded

3. Review the model

• Review the model the seeder class links to.
• All fields on the model should also be on the seeder.

4. Build the fields dictionary

• Decide the field types needed to build the most realistic data structures.

5. Custom Methods Usage

Leverage built-in custom methods for common patterns:

• in_order: Sequential value assignment for maintaining data relationships
• date_time_between: Random date generation within ranges
• fk_random: Random foreign key selection from existing records
• fk_in_order: Sequential foreign key assignment for maintaining referential integrity

6. Module seed.py

• Update the seed.py field for the module
• Use seed() for object instantiation without DB insertion
• Use seed_database() for direct database insertion
• Combine with field overrides for edge case testing
• When overriding fields in seed methods, you can customize base seeders for specific use cases

7. Project seed.py

• Ensure the seed.py we just configured is important in the project root seed.py

Edge Case Seeding

Override fields to create specific test scenarios:

ProductSeeder.seed(
    count=1,
    fields={"in_stock":  False}
)

Create specialized class methods that need to be re-used:

class ProductSeeder(DjangoModelSeeder):
    @classmethod
    def seed_grocery_product(cls, count: int = 1):
        cls.seed_database(
            count=count,
            fields=cls.fields | {
                'name': ('llm', 'Grocery product name'),
                'description': ('llm', 'Grocery product description'),
            }
        ) 

Seeding in specific orders

• When building class methods, chain calls down the dependency hierarchy to keep modularity.
• Each Seeder class is responsible for creating its model.
• In the example below, I can call ProductSeeder.seed_with_inventory(count=20) and it generates all the inventory batches and inventory objects with the correct relationships.
• Only the parent seeder will be in our project root seed.py.

# Product Seeder needs to link to batches and inventory

class ProductSeeder(DjangoModelSeeder):
    ... 
    
    @classmethod
    def seed_with_inventory(cls, count: int = 1):
        products = cls.seed_database(count=count)

        for product in products:
            BatchSeeder.seed_by_product(
                product=product,
                count=2,
            )

        return products



class BatchSeeder(DjangoModelSeeder):
    ... 
    @classmethod
    def seed_by_product(
            cls,
            product: Product,
            count: int = 1
    ):
        batches =  cls.seed_database(
            count=count,
            fields={
                'product_id': product.id
            }
        )

        for batch in batches:
            InventorySeeder.seed_database(
                count=3,
                fields={
                    'product_id': product.id,
                    'batch_id': batch.id
                }
            )

        return batches
    
    class InventorySeeder(DjangoModelSeeder):
     ...
    @classmethod
    def seed_by_product_and_batch(cls, product, batch, count: int = 1):
        """
        Seed inventory items specifically for a given product and batch.
        This ensures proper relationship with both product and batch.
        """
        return cls.seed_database(
            count=count,
            fields={
                'product_id': product.id,
                'batch_id': batch.id
            }
        )

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