fix(bun): support extra generated model fields

pkg/writers/bun/template_data.go

@@ -1,6 +1,8 @@
 package bun
 
 import (
+	"encoding/json"
+	"fmt"
 	"sort"
 	"strings"
 
@@ -32,6 +34,90 @@ type ModelData struct {
 	PrimaryKeyIDType string // Helper method GetID/SetID/UpdateID type
 	IDColumnName     string // Name of the ID column in database
 	Prefix           string // 3-letter prefix
+
+	// ExtraFields are user-defined fields added via generator config (issue #4)
+	ExtraFields []*FieldData `json:"extra_fields,omitempty"`
+}
+
+// ExtraFieldConfig represents a custom field to be added to generated models
+type ExtraFieldConfig struct {
+	Name        string `json:"name"`
+	Type        string `json:"type"`
+	BunTag      string `json:"bun_tag,omitempty"`
+	JSONTag     string `json:"json_tag,omitempty"`
+	Comment     string `json:"comment,omitempty"`
+	TargetTable string `json:"target_table,omitempty"` // optional: if set, field only applies to this table
+}
+
+// LoadExtraFieldsFromMetadata loads custom field configurations from metadata map.
+// Accepts either a JSON-encoded array of fields (for CLI use) or a structured list
+// (for programmatic/sidecar file use). Each entry must have "name" and "type";
+// an optional "target_table" scopes the field to one table only.
+func LoadExtraFieldsFromMetadata(metadata map[string]interface{}) []ExtraFieldConfig {
+	if metadata == nil {
+		return nil
+	}
+
+	extraFieldsRaw, ok := metadata["extra_fields"]
+	if !ok || extraFieldsRaw == nil {
+		return nil
+	}
+
+	// Try to parse as JSON string first (from CLI flag)
+	if strVal, ok := extraFieldsRaw.(string); ok {
+		var fields []ExtraFieldConfig
+		if err := json.Unmarshal([]byte(strVal), &fields); err == nil && len(fields) > 0 {
+			return filterValidFields(fields)
+		}
+	}
+
+	// Try to parse as structured data (from sidecar file or programmatic API)
+	extraFieldsList, ok := extraFieldsRaw.([]interface{})
+	if !ok || len(extraFieldsList) == 0 {
+		return nil
+	}
+
+	fields := make([]ExtraFieldConfig, 0, len(extraFieldsList))
+	for _, item := range extraFieldsList {
+		if fieldMap, ok := item.(map[string]interface{}); ok {
+			field := ExtraFieldConfig{
+				Name:        toString(fieldMap["name"]),
+				Type:        toString(fieldMap["type"]),
+				BunTag:      toString(fieldMap["bun_tag"]),
+				JSONTag:     toString(fieldMap["json_tag"]),
+				Comment:     toString(fieldMap["comment"]),
+				TargetTable: toString(fieldMap["target_table"]),
+			}
+			if field.Name != "" && field.Type != "" {
+				fields = append(fields, field)
+			}
+		}
+	}
+
+	return filterValidFields(fields)
+}
+
+// filterValidFields removes entries that are missing required fields.
+func filterValidFields(fields []ExtraFieldConfig) []ExtraFieldConfig {
+	var valid []ExtraFieldConfig
+	for _, f := range fields {
+		if f.Name != "" && f.Type != "" {
+			valid = append(valid, f)
+		}
+	}
+	return valid
+}
+
+func toString(v interface{}) string {
+	if v == nil {
+		return ""
+	}
+	switch val := v.(type) {
+	case string:
+		return val
+	default:
+		return fmt.Sprintf("%v", val)
+	}
 }
 
 // FieldData represents a single field in a struct