组合关系中子类是否必须包含ID字段？数据库表转类图困惑及冗余验证请求

Let’s break down your two core questions with practical, context-specific advice:

1. Do "Child" Classes in Composition Relationships Need an ID Field? #

First, let’s clarify: in composition (a strict "whole-part" relationship where parts can’t exist without their parent), there’s no mandatory rule that requires the "part" class to have its own unique ID. It all depends on your business needs:

• Your PurchaseFinisher design is totally valid: Using id_purchase + id_payment_method as a composite key directly reflects its dependency on Purchase—without a corresponding Purchase, a PurchaseFinisher has no reason to exist. This is a common pattern for parts that don’t need to be referenced independently by other entities.
• PurchaseItem having an ID is also fine: A standalone ID makes sense if you need to track the PurchaseItem individually (e.g., for audit logs, or if another entity like ReturnRequest needs to link directly to a specific PurchaseItem). But this isn’t a requirement for composition—it’s just a design choice based on use cases.

In short: Composition only enforces the lifecycle dependency, not the way you identify the part. Both approaches are acceptable.

2. Spotting Redundancy Between Purchase and Product #

Since you’re confident in your database model, the redundancy is likely creeping into your class definitions. Here are the most common scenarios to check:

• Duplicate attribute storage in Purchase: If your Purchase class (or its database table) holds Product-specific fields like product_name or base_price that should only live in the Product class, that’s redundancy. Even if you’re storing a snapshot of the product’s price at purchase time, that data belongs in PurchaseItem (as a line-item detail), not directly in Purchase.
• Unnecessary intermediate class (or missing purpose for one): If your PurchaseItem class only links Purchase and Product with no additional attributes (like quantity, discount, or line_total), it’s redundant. In this case, Purchase and Product should have a direct many-to-many association. But if PurchaseItem does include those extra details, it’s a necessary part of the model—no redundancy here.
• Dual association paths: If your class diagram shows Purchase linking to Product both directly (e.g., a Product property in Purchase) and indirectly via PurchaseItem, that’s redundant. This creates two sources of truth for the same relationship, which can lead to inconsistencies. For example, if you update the direct Product link but forget to update PurchaseItem, your model will be out of sync.

A quick example: Suppose your database uses PurchaseItem to track multiple products per order, but in your class definition, you added a List<Product> property to Purchase alongside List<PurchaseItem>. That’s redundant—all product-related info should be accessed through PurchaseItem, which already ties the product to the order with any necessary line-item details.

Wrap-Up #

If you can share a bit more detail about your database table fields or class structure, I can pinpoint the exact redundancy. But based on common e-commerce modeling patterns, these are the most likely culprits.

内容的提问来源于stack exchange，提问作者Alex Boorjs