GORM Polymorphic: Wrong Association Queries

In this tutorial, you'll learn about GORM Polymorphic: Wrong Association Queries. We cover key concepts, practical examples, and best practices.

Polymorphic associations in GORM -- Allow a model to belong to multiple other models using a single association.

The Problem

Polymorphic associations let a model (Image) belong to multiple models (User, Product) through type and ID columns.

Wrong

type Image struct {
    ID   uint
    URL  string
    UserID uint // Belongs to User only
}

Output:

// Image can only belong to User. Need separate table for Product.

Right

type Image struct {
    ID        uint
    URL       string
    OwnerID   uint
    OwnerType string
}
type User struct {
    Images []Image `gorm:"polymorphic:Owner"`
}
type Product struct {
    Images []Image `gorm:"polymorphic:Owner"`
}

Output:

// Images table: owner_id, owner_type = "users" or "products"
// Single table works for both

Prevention

• Use gorm:"polymorphic:Owner" on slice field
• GORM creates owner_id and owner_type columns
• Owner type stores parent table name
• Query across parent types with Preload
• Supports CRUD like other associations

Common Mistakes with gorm polymorphic

1. Forgetting deriving (Show, Eq) on custom data types needed for debugging
2. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable
3. Using head and tail instead of pattern matching, causing runtime errors on empty lists

These mistakes appear frequently in real-world GO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

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