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Items Domain

Overview​

The Items domain is the foundational master data module for the inventory system, managing all product and item catalog information. It uses a composition-based architecture pattern that provides maximum flexibility for different item types without rigid inheritance hierarchies.

Core Concepts​

Composition Over Inheritance​

The Items domain adopts a composition-based architecture instead of traditional class inheritance. This design decision enables:

  • Flexibility: Items can have behaviors added or removed dynamically
  • Simplicity: No complex inheritance hierarchies to manage
  • Extensibility: Easy to add new behaviors without modifying existing code
  • Clear Separation: Each behavior is self-contained and testable

Item Classification​

Items in the system fall into two main categories based on behavior:

  1. Stockable Items: Items that have inventory tracking enabled via StockableBehavior

    • Examples: Raw materials, finished goods, components
    • Tracked across locations and transactions
    • Support negative stock control (configurable)
    • Location-based tracking capabilities

  2. Non-Stockable Items: Items without inventory tracking

    • Examples: Services, labor, intangible items
    • No inventory quantities maintained
    • Used for reference and billing purposes only

Unit Class System​

Instead of forcing items to use specific units of measure, the Items domain uses a UnitClass system:

  • Items reference a UnitClass (e.g., "Weight", "Length", "Volume")
  • Transactions can use any UnitOfMeasure within that class
  • Example: Item uses "Weight" class → transactions can use kg, lb, g, oz, tons, etc.
  • Provides maximum flexibility at transaction time

Domain Structure​

Aggregates​

items/
├── aggregates/
│   └── item.aggregate.md          # Core Item aggregate documentation

Entities​

items/
└── entities/
    ├── behaviors/
    │   └── stockable-behavior.md  # Stockable behavior composition
    └── item.md                     # Item entity (aggregate root)

Value Objects​

The Items domain currently uses simple value types. Future enhancements may include:

  • Item pricing value objects
  • Dimension specifications (size, weight, color)
  • Product identifiers (SKU, UPC, EAN)

Key Aggregates​

Item​

Aggregate Root: Item

Purpose: Core catalog entity representing any product, material, or service in the system.

Key Characteristics:

  • Uses composition pattern with optional behaviors
  • References category for hierarchical classification
  • Uses UnitClass for flexible measurement
  • Supports tax group integration
  • Maintains item lifecycle (active/inactive)

Relationships:

  • Belongs to one Category (optional)
  • References one UnitClass (optional)
  • References one TaxGroup (optional)
  • Can have one StockableBehavior (optional composition)

Core Operations:

  • Create item with basic properties
  • Add/remove/update stockable behavior
  • Update item information
  • Category assignment
  • Tax group assignment

Business Rules​

Item Creation Rules​

  1. Item Number Required: Every item must have a unique item number (identifier)
  2. Name Required: Item name cannot be empty
  3. Description Optional: Description provides additional details but is not mandatory
  4. Category Optional: Items can exist without category assignment
  5. Unit Class Optional: Items can be created without a unit class (assigned later)

Stockable Behavior Rules​

  1. Single Behavior: An item can have at most one stockable behavior at a time
  2. Cannot Duplicate: Cannot add stockable behavior if already present
  3. Cannot Remove Missing: Cannot remove stockable behavior that doesn't exist
  4. Negative Stock Control: Each stockable item configures whether negative stock is allowed
  5. Location Tracking: Each stockable item configures whether location-based tracking is required

Update Rules​

  1. Immutable Item Number: Item number cannot be changed after creation
  2. Name Cannot Be Empty: Updates must provide valid item name
  3. Behavior Updates: Stockable behavior settings can be updated independently
  4. Category Reassignment: Category can be changed or cleared at any time
  5. Tax Group Flexibility: Tax group assignment can be updated as needed

Integration Points​

With Transaction Module​

  • Transaction Creation: Validates that item is stockable for inventory transactions
  • Negative Stock Validation: Checks item's negative stock setting before allowing negative inventory
  • Location Tracking: Enforces location requirements based on item's tracking settings
  • Unit Validation: Ensures transaction units match item's unit class

With Inventory Module​

  • Inventory Records: Only stockable items have inventory records
  • Stock Queries: Non-stockable items return zero or no inventory data
  • Location-Based Tracking: Inventory tracks per-location if item requires it

With BOM Module​

  • Parent Items: Items can be parent items in BOMs (finished goods)
  • Component Items: Items can be components in BOMs (raw materials)
  • Assembly Validation: Verifies items are stockable for assembly operations

With Finance Module​

  • Tax Group Integration: Item tax group used for financial transaction tax calculations
  • Revenue/COGS Accounts: Item properties used to determine GL accounts for posting
  • Integration Events: Purchase/sale events trigger inventory updates for stockable items

Common Patterns​

Creating Stockable Items​

// 1. Create basic item
var item = Item.Create(
    itemNumber: "WIDGET-001",
    name: "Premium Widget",
    description: "High-quality widget for manufacturing",
    categoryId: widgetCategoryId,
    defaultUnitClassId: weightUnitClassId,
    taxGroupId: standardTaxGroupId);

// 2. Add stockable behavior to enable inventory tracking
item.AddStockableBehavior(
    allowNegativeStock: false,     // Prevent negative inventory
    trackByLocation: true);         // Require location for transactions

// 3. Save via repository
await itemRepository.AddAsync(item);
await unitOfWork.CommitAsync();

Creating Non-Stockable Items (Services)​

// Create item without adding stockable behavior
var serviceItem = Item.Create(
    itemNumber: "SRV-INSTALL",
    name: "Installation Service",
    description: "Professional installation service",
    categoryId: servicesCategoryId);

// No stockable behavior added - item remains non-stockable

await itemRepository.AddAsync(serviceItem);
await unitOfWork.CommitAsync();

Converting Between Stockable and Non-Stockable​

// Make item stockable
item.AddStockableBehavior(
    allowNegativeStock: true,
    trackByLocation: true);

// Later, make it non-stockable
// (Note: Should validate no inventory exists first)
item.RemoveStockableBehavior();

Updating Stockable Settings​

// Update behavior settings without removing/re-adding
item.UpdateStockableBehavior(
    allowNegativeStock: true,      // Now allow negative stock
    trackByLocation: false);        // Disable location tracking

Repository Pattern​

IItemRepository​

Query Methods:

  • GetByIdAsync(Guid id): Get item with all related data
  • GetAllAsync(): Get all active items
  • GetByItemNumberAsync(string itemNumber): Find item by unique number
  • GetStockableItemsAsync(): Get only items with stockable behavior
  • GetByCategoryAsync(Guid categoryId): Get items in specific category

Command Methods:

  • AddAsync(Item item): Create new item
  • Update(Item item): Update existing item
  • Archive(Item item): Soft delete (sets IsActive = false)
  • UnArchive(Item item): Restore archived item

Validation Methods:

  • ExistsByItemNumberAsync(string itemNumber): Check uniqueness
  • IsItemNumberUniqueAsync(string itemNumber, Guid? excludeId): Validate for updates

Domain Services​

IItemService​

Purpose: Coordinates complex operations involving multiple aggregates.

Key Methods:

  • ValidateItemForTransactionAsync(Guid itemId, TransactionType type): Ensures item can be used in transaction
  • GetAvailableUnitsForItemAsync(Guid itemId): Gets valid units based on item's unit class
  • ConvertStockableStatusAsync(Guid itemId, bool makeStockable): Safely converts with validation

Testing Strategy​

Unit Tests​

  • Factory Method Tests: Validate item creation with various combinations
  • Behavior Tests: Add/remove/update stockable behavior scenarios
  • Validation Tests: Business rule enforcement (required fields, duplicates)
  • Update Tests: Property updates and constraint validation

Integration Tests​

  • Repository Tests: Database operations and query correctness
  • Relationship Tests: Category, UnitClass, TaxGroup associations
  • Behavior Persistence: Stockable behavior correctly saved/loaded
  • Soft Delete Tests: Archive/UnArchive functionality

Domain Event Tests​

Future enhancement when domain events are added for:

  • Item created events
  • Stockable behavior changed events
  • Item archived/restored events

Future Enhancements​

Planned Features​

  1. Item Variants: Support for size/color/style variants of a base item
  2. Item Attributes: Flexible attribute system for product specifications
  3. Pricing Management: Built-in price lists and pricing tiers
  4. Supplier Relationships: Link items to preferred suppliers
  5. Item Images: Support for product images and documents
  6. Serial/Batch Tracking: Enhanced tracking for lot-controlled items

Extensibility Points​

  1. Additional Behaviors: New behaviors can be added via composition

    • PurchasableBehavior: Purchase-specific rules
    • SellableBehavior: Sales-specific rules
    • ManufacturableBehavior: Production-specific rules

  2. Custom Validation: Domain service can be extended for business-specific rules

  3. Integration Events: Publish events for cross-module integration

    • ItemCreated
    • ItemStockableStatusChanged
    • ItemCategoryChanged

Domain Documentation​

API Documentation​

  • Items API - REST endpoints for item management

Concept Documentation​

Architecture Documentation​

Last Updated: 2025-10-24 | Status: Active Development | Version: 1.0