Better handling of preloads

pkg/common/adapters/database/RELATION_LOADING.md

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+# Automatic Relation Loading Strategies
+
+## Overview
+
+**NEW:** The database adapters now **automatically** choose the optimal loading strategy by inspecting your model's relationship tags!
+
+Simply use `PreloadRelation()` and the system automatically:
+- Detects relationship type from Bun/GORM tags
+- Uses **JOIN** for many-to-one and one-to-one (efficient, no duplication)
+- Uses **separate query** for one-to-many and many-to-many (avoids duplication)
+
+## How It Works
+
+```go
+// Just write this - the system handles the rest!
+db.NewSelect().
+    Model(&links).
+    PreloadRelation("Provider").  // ✓ Auto-detects belongs-to → uses JOIN
+    PreloadRelation("Tags").      // ✓ Auto-detects has-many → uses separate query
+    Scan(ctx, &links)
+```
+
+### Detection Logic
+
+The system inspects your model's struct tags:
+
+**Bun models:**
+```go
+type Link struct {
+    Provider   *Provider `bun:"rel:belongs-to"`   // → Detected: belongs-to → JOIN
+    Tags       []Tag     `bun:"rel:has-many"`     // → Detected: has-many → Separate query
+}
+```
+
+**GORM models:**
+```go
+type Link struct {
+    ProviderID int
+    Provider   *Provider `gorm:"foreignKey:ProviderID"`  // → Detected: belongs-to → JOIN
+    Tags       []Tag     `gorm:"many2many:link_tags"`    // → Detected: many-to-many → Separate query
+}
+```
+
+**Type inference (fallback):**
+- `[]Type` (slice) → has-many → Separate query
+- `*Type` (pointer) → belongs-to → JOIN
+- `Type` (struct) → belongs-to → JOIN
+
+### What Gets Logged
+
+Enable debug logging to see strategy selection:
+
+```go
+bunAdapter.EnableQueryDebug()
+```
+
+**Output:**
+```
+DEBUG: PreloadRelation 'Provider' detected as: belongs-to
+INFO:  Using JOIN strategy for belongs-to relation 'Provider'
+DEBUG: PreloadRelation 'Links' detected as: has-many
+DEBUG: Using separate query for has-many relation 'Links'
+```
+
+## Relationship Types
+
+| Bun Tag | GORM Pattern | Field Type | Strategy | Why |
+|---------|--------------|------------|----------|-----|
+| `rel:has-many` | Slice field | `[]Type` | Separate Query | Avoids duplicating parent data |
+| `rel:belongs-to` | `foreignKey:` | `*Type` | JOIN | Single parent, no duplication |
+| `rel:has-one` | Single pointer | `*Type` | JOIN | One-to-one, no duplication |
+| `rel:many-to-many` | `many2many:` | `[]Type` | Separate Query | Complex join, avoid cartesian |
+
+## Manual Override
+
+If you need to force a specific strategy, use `JoinRelation()`:
+
+```go
+// Force JOIN even for has-many (not recommended)
+db.NewSelect().
+    Model(&providers).
+    JoinRelation("Links").  // Explicitly use JOIN
+    Scan(ctx, &providers)
+```
+
+## Examples
+
+### Automatic Strategy Selection (Recommended)
+
+```go
+// Example 1: Loading parent provider for each link
+// System detects belongs-to → uses JOIN automatically
+db.NewSelect().
+    Model(&links).
+    PreloadRelation("Provider", func(q common.SelectQuery) common.SelectQuery {
+        return q.Where("active = ?", true)
+    }).
+    Scan(ctx, &links)
+
+// Generated SQL: Single query with JOIN
+// SELECT links.*, providers.*
+// FROM links
+// LEFT JOIN providers ON links.provider_id = providers.id
+// WHERE providers.active = true
+
+// Example 2: Loading child links for each provider
+// System detects has-many → uses separate query automatically
+db.NewSelect().
+    Model(&providers).
+    PreloadRelation("Links", func(q common.SelectQuery) common.SelectQuery {
+        return q.Where("active = ?", true)
+    }).
+    Scan(ctx, &providers)
+
+// Generated SQL: Two queries
+// Query 1: SELECT * FROM providers
+// Query 2: SELECT * FROM links
+//          WHERE provider_id IN (1, 2, 3, ...)
+//          AND active = true
+```
+
+### Mixed Relationships
+
+```go
+type Order struct {
+    ID         int
+    CustomerID int
+    Customer   *Customer `bun:"rel:belongs-to"`    // JOIN
+    Items      []Item    `bun:"rel:has-many"`      // Separate
+    Invoice    *Invoice  `bun:"rel:has-one"`       // JOIN
+}
+
+// All three handled optimally!
+db.NewSelect().
+    Model(&orders).
+    PreloadRelation("Customer").  // → JOIN (many-to-one)
+    PreloadRelation("Items").     // → Separate (one-to-many)
+    PreloadRelation("Invoice").   // → JOIN (one-to-one)
+    Scan(ctx, &orders)
+```
+
+## Performance Benefits
+
+### Before (Manual Strategy Selection)
+
+```go
+// You had to remember which to use:
+.PreloadRelation("Provider")  // Should I use PreloadRelation or JoinRelation?
+.PreloadRelation("Links")     // Which is more efficient here?
+```
+
+### After (Automatic Selection)
+
+```go
+// Just use PreloadRelation everywhere:
+.PreloadRelation("Provider")  // ✓ System uses JOIN automatically
+.PreloadRelation("Links")     // ✓ System uses separate query automatically
+```
+
+## Migration Guide
+
+**No changes needed!** If you're already using `PreloadRelation()`, it now automatically optimizes:
+
+```go
+// Before: Always used separate query
+.PreloadRelation("Provider")  // Inefficient: extra round trip
+
+// After: Automatic optimization
+.PreloadRelation("Provider")  // ✓ Now uses JOIN automatically!
+```
+
+## Implementation Details
+
+### Supported Bun Tags
+- `rel:has-many` → Separate query
+- `rel:belongs-to` → JOIN
+- `rel:has-one` → JOIN
+- `rel:many-to-many` or `rel:m2m` → Separate query
+
+### Supported GORM Patterns
+- `many2many:` tag → Separate query
+- `foreignKey:` tag → JOIN (belongs-to)
+- `[]Type` slice without many2many → Separate query (has-many)
+- `*Type` pointer with foreignKey → JOIN (belongs-to)
+- `*Type` pointer without foreignKey → JOIN (has-one)
+
+### Fallback Behavior
+- `[]Type` (slice) → Separate query (safe default for collections)
+- `*Type` or `Type` (single) → JOIN (safe default for single relations)
+- Unknown → Separate query (safest default)
+
+## Debugging
+
+To see strategy selection in action:
+
+```go
+// Enable debug logging
+bunAdapter.EnableQueryDebug()  // or gormAdapter.EnableQueryDebug()
+
+// Run your query
+db.NewSelect().
+    Model(&records).
+    PreloadRelation("RelationName").
+    Scan(ctx, &records)
+
+// Check logs for:
+// - "PreloadRelation 'X' detected as: belongs-to"
+// - "Using JOIN strategy for belongs-to relation 'X'"
+// - Actual SQL queries executed
+```
+
+## Best Practices
+
+1. **Use PreloadRelation() for everything** - Let the system optimize
+2. **Define proper relationship tags** - Ensures correct detection
+3. **Only use JoinRelation() for overrides** - When you know better than auto-detection
+4. **Enable debug logging during development** - Verify optimal strategies are chosen
+5. **Trust the system** - It's designed to choose correctly based on relationship type