fix(reflection): update GetForeignKeyColumn to return multiple columns

* Change return type to []string for composite keys
* Adjust related logic in injectForeignKeys method
* Update tests to validate new behavior for composite foreign keys

pkg/common/recursive_crud.go

@@ -235,9 +235,9 @@ func (p *NestedCUDProcessor) injectForeignKeys(data map[string]interface{}, mode
 	}
 
 	for parentKey, parentID := range parentIDs {
-		dbColName := reflection.GetForeignKeyColumn(modelType, parentKey)
+		dbColNames := reflection.GetForeignKeyColumn(modelType, parentKey)
 
-		if dbColName == "" {
+		if len(dbColNames) == 0 {
 			// No explicit tag found — fall back to naming convention by scanning scalar fields.
 			for i := 0; i < modelType.NumField(); i++ {
 				field := modelType.Field(i)
@@ -248,13 +248,13 @@ func (p *NestedCUDProcessor) injectForeignKeys(data map[string]interface{}, mode
 					strings.EqualFold(jsonName, parentKey+"_id") ||
 					strings.EqualFold(jsonName, parentKey+"id") ||
 					strings.EqualFold(field.Name, parentKey+"ID") {
-					dbColName = reflection.GetColumnName(field)
+					dbColNames = []string{reflection.GetColumnName(field)}
 					break
 				}
 			}
 		}
 
-		if dbColName != "" {
+		for _, dbColName := range dbColNames {
 			if _, exists := data[dbColName]; !exists {
 				logger.Debug("Injecting foreign key: %s = %v", dbColName, parentID)
 				data[dbColName] = parentID

pkg/reflection/model_utils.go

@@ -973,21 +973,23 @@ func GetRelationType(model interface{}, fieldName string) RelationType {
 	return RelationUnknown
 }
 
-// GetForeignKeyColumn returns the DB column name of the foreign key that the
+// GetForeignKeyColumn returns the DB column names of the foreign key(s) that
-// relation field identified by parentKey owns on modelType.
+// the relation field identified by parentKey owns on modelType. Composite keys
+// (e.g. bun "join:a=b,join:c=d" or GORM "foreignKey:ColA,ColB") yield multiple
+// entries. Returns nil when no tag is found (caller should fall back to
+// convention).
 //
 // It checks tags in priority order:
-//  1. Bun join: tag — e.g. `bun:"rel:belongs-to,join:department_id=id"` → "department_id"
+//  1. Bun join: tag — e.g. `bun:"rel:belongs-to,join:department_id=id"` → ["department_id"]
-//  2. GORM foreignKey: tag — e.g. `gorm:"foreignKey:DepartmentID"` → column of DepartmentID field
+//  2. GORM foreignKey: tag — e.g. `gorm:"foreignKey:DepartmentID"` → [column of DepartmentID field]
-//  3. Returns "" when no tag is found (caller should fall back to convention)
 //
 // parentKey is matched case-insensitively against the field name and JSON tag.
-func GetForeignKeyColumn(modelType reflect.Type, parentKey string) string {
+func GetForeignKeyColumn(modelType reflect.Type, parentKey string) []string {
 	for modelType.Kind() == reflect.Ptr || modelType.Kind() == reflect.Slice {
 		modelType = modelType.Elem()
 	}
 	if modelType.Kind() != reflect.Struct {
-		return ""
+		return nil
 	}
 
 	for i := 0; i < modelType.NumField(); i++ {
@@ -999,34 +1001,42 @@ func GetForeignKeyColumn(modelType reflect.Type, parentKey string) string {
 			continue
 		}
 
-		// Bun: join:local_col=foreign_col
+		// Bun: join:local_col=foreign_col (one join: part per pair)
+		var bunCols []string
 		for _, part := range strings.Split(field.Tag.Get("bun"), ",") {
 			part = strings.TrimSpace(part)
 			if strings.HasPrefix(part, "join:") {
-				// join: may contain multiple pairs separated by spaces: "join:a=b join:c=d"
-				// but typically it's a single pair; take the first local column
 				pair := strings.TrimPrefix(part, "join:")
 				if idx := strings.Index(pair, "="); idx > 0 {
-					return pair[:idx]
+					bunCols = append(bunCols, pair[:idx])
 				}
 			}
 		}
+		if len(bunCols) > 0 {
+			return bunCols
+		}
 
-		// GORM: foreignKey:FieldName
+		// GORM: foreignKey:FieldA,FieldB
 		for _, part := range strings.Split(field.Tag.Get("gorm"), ";") {
 			part = strings.TrimSpace(part)
 			if strings.HasPrefix(part, "foreignKey:") {
-				fkFieldName := strings.TrimPrefix(part, "foreignKey:")
+				var cols []string
+				for _, fkFieldName := range strings.Split(strings.TrimPrefix(part, "foreignKey:"), ",") {
+					fkFieldName = strings.TrimSpace(fkFieldName)
 					if fkField, ok := modelType.FieldByName(fkFieldName); ok {
-					return getColumnNameFromField(fkField)
+						cols = append(cols, getColumnNameFromField(fkField))
+					}
+				}
+				if len(cols) > 0 {
+					return cols
 				}
 			}
 		}
 
-		return ""
+		return nil
 	}
 
-	return ""
+	return nil
 }
 
 // GetRelationModel gets the model type for a relation field

pkg/reflection/model_utils_foreign_key_test.go

@@ -15,6 +15,13 @@ type bunEmployee struct {
 	Department *fkDept  `bun:"rel:belongs-to,join:dept_id=id"   json:"department"`
 }
 
+// bunCompositeEmployee has a composite bun join: (two join: parts).
+type bunCompositeEmployee struct {
+	DeptID   string  `bun:"dept_id"   json:"dept_id"`
+	TenantID string  `bun:"tenant_id" json:"tenant_id"`
+	Department *fkDept `bun:"rel:belongs-to,join:dept_id=id,join:tenant_id=id" json:"department"`
+}
+
 // gormEmployee uses gorm foreignKey: tag (mirrors testmodels.Employee).
 type gormEmployee struct {
 	DepartmentID string  `json:"department_id"`
@@ -23,6 +30,13 @@ type gormEmployee struct {
 	Manager      *fkDept `gorm:"foreignKey:ManagerID;references:ID"    json:"manager"`
 }
 
+// gormCompositeEmployee has a composite GORM foreignKey.
+type gormCompositeEmployee struct {
+	DeptID   string  `json:"dept_id"`
+	TenantID string  `json:"tenant_id"`
+	Department *fkDept `gorm:"foreignKey:DeptID,TenantID" json:"department"`
+}
+
 // conventionEmployee has no explicit FK tag — relies on naming convention.
 type conventionEmployee struct {
 	DepartmentID string  `json:"department_id"`
@@ -39,20 +53,26 @@ func TestGetForeignKeyColumn(t *testing.T) {
 		name      string
 		modelType reflect.Type
 		parentKey string
-		want      string
+		want      []string
 	}{
 		// Bun join: tag
 		{
 			name:      "bun join tag returns local column",
 			modelType: reflect.TypeOf(bunEmployee{}),
 			parentKey: "department",
-			want:      "dept_id",
+			want:      []string{"dept_id"},
 		},
 		{
 			name:      "bun join tag matched via json tag (case-insensitive)",
 			modelType: reflect.TypeOf(bunEmployee{}),
 			parentKey: "Department",
-			want:      "dept_id",
+			want:      []string{"dept_id"},
+		},
+		{
+			name:      "bun composite join returns all local columns",
+			modelType: reflect.TypeOf(bunCompositeEmployee{}),
+			parentKey: "department",
+			want:      []string{"dept_id", "tenant_id"},
 		},
 
 		// GORM foreignKey: tag
@@ -60,19 +80,25 @@ func TestGetForeignKeyColumn(t *testing.T) {
 			name:      "gorm foreignKey resolves to column name",
 			modelType: reflect.TypeOf(gormEmployee{}),
 			parentKey: "department",
-			want:      "department_id",
+			want:      []string{"department_id"},
 		},
 		{
 			name:      "gorm foreignKey resolves second relation",
 			modelType: reflect.TypeOf(gormEmployee{}),
 			parentKey: "manager",
-			want:      "manager_id",
+			want:      []string{"manager_id"},
 		},
 		{
 			name:      "gorm foreignKey matched case-insensitively",
 			modelType: reflect.TypeOf(gormEmployee{}),
 			parentKey: "Department",
-			want:      "department_id",
+			want:      []string{"department_id"},
+		},
+		{
+			name:      "gorm composite foreignKey returns all columns",
+			modelType: reflect.TypeOf(gormCompositeEmployee{}),
+			parentKey: "department",
+			want:      []string{"dept_id", "tenant_id"},
 		},
 
 		// Pointer and slice unwrapping
@@ -80,43 +106,43 @@ func TestGetForeignKeyColumn(t *testing.T) {
 			name:      "pointer to struct is unwrapped",
 			modelType: reflect.TypeOf(&gormEmployee{}),
 			parentKey: "department",
-			want:      "department_id",
+			want:      []string{"department_id"},
 		},
 		{
 			name:      "slice of struct is unwrapped",
 			modelType: reflect.TypeOf([]gormEmployee{}),
 			parentKey: "department",
-			want:      "department_id",
+			want:      []string{"department_id"},
 		},
 
-		// No tag — returns "" so caller can fall back to convention
+		// No tag — returns nil so caller can fall back to convention
 		{
-			name:      "relation with no FK tag returns empty string",
+			name:      "relation with no FK tag returns nil",
 			modelType: reflect.TypeOf(conventionEmployee{}),
 			parentKey: "department",
-			want:      "",
+			want:      nil,
 		},
 
 		// Unknown parent key
 		{
-			name:      "unknown parent key returns empty string",
+			name:      "unknown parent key returns nil",
 			modelType: reflect.TypeOf(gormEmployee{}),
 			parentKey: "nonexistent",
-			want:      "",
+			want:      nil,
 		},
 		{
-			name:      "non-struct type returns empty string",
+			name:      "non-struct type returns nil",
 			modelType: reflect.TypeOf(""),
 			parentKey: "department",
-			want:      "",
+			want:      nil,
 		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			got := GetForeignKeyColumn(tt.modelType, tt.parentKey)
-			if got != tt.want {
+			if !reflect.DeepEqual(got, tt.want) {
-				t.Errorf("GetForeignKeyColumn(%v, %q) = %q, want %q", tt.modelType, tt.parentKey, got, tt.want)
+				t.Errorf("GetForeignKeyColumn(%v, %q) = %v, want %v", tt.modelType, tt.parentKey, got, tt.want)
 			}
 		})
 	}