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/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 {
-				t.Errorf("GetForeignKeyColumn(%v, %q) = %q, want %q", tt.modelType, tt.parentKey, got, tt.want)
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("GetForeignKeyColumn(%v, %q) = %v, want %v", tt.modelType, tt.parentKey, got, tt.want)
 			}
 		})
 	}