Usage Of Edge Schema In Other Edge Types but more indexes.

I have two entity, first one is **User**:
```go
// User holds the schema definition for the User entity.
type User struct {
ent.Schema
}

// Annotation of the User.
func (User) Annotations() []schema.Annotation {
return []schema.Annotation{
entsql.Annotation{Table: "user"},
entsql.WithComments(true),
}
}

// Fields of the User.
func (User) Fields() []ent.Field {
return []ent.Field{
field.String("nickname").NotEmpty(),
...
}
}

// Edges of the User.
func (User) Edges() []ent.Edge {
return []ent.Edge{
edge.To("took_exams", Exam.Type).Through("exam_results", ExamResult.Type),
}
}
```
second one is **Exam**
```go
// Exam holds the schema definition for the Exam entity.
type Exam struct {
ent.Schema
}

// Annotation of the Exam.
func (Exam) Annotations() []schema.Annotation {
return []schema.Annotation{
entsql.Annotation{Table: "exam"},
entsql.WithComments(true),
}
}

// Fields of the Exam.
func (Exam) Fields() []ent.Field {
return []ent.Field{
field.String("name").NotEmpty(),
...
}
}

// Edges of the Exam.
func (Exam) Edges() []ent.Edge {
return []ent.Edge{
edge.From("exam_users", User.Type).Ref("took_exams").Through("exam_results", ExamResult.Type),
}
}
```
User can take exam more then 1 time, so I have a separate table to record **user exam histories** :
```go
// ExamResult holds the schema definition for the ExamResult entity.
type ExamResult struct {
ent.Schema
}

// Annotations of the ExamResult.
func (ExamResult) Annotations() []schema.Annotation {
return []schema.Annotation{
field.ID("user_id", "exam_id", "times"),
entsql.Table("exam_result"),
entsql.WithComments(true),
}
}

// Fields of the ExamResult.
func (ExamResult) Fields() []ent.Field {
return []ent.Field{
field.Int("user_id"),
field.Int("exam_id"),
field.Int("times"),
field.Int("score"),
...
}
}

// Edges of the ExamResult.
func (ExamResult) Edges() []ent.Edge {
return []ent.Edge{
edge.To("user", User.Type).Field("user_id").Unique().Required(),
edge.To("exam", Exam.Type).Field("exam_id").Unique().Required(),
}
}
```

I want to use `user_id` 、`exam_id` and `times` to be indexes. but when I run `generate` error comes out `edge schema primary key can only be defined on "id" or ("user_id", "exam_id") in the same order` .
I don't know what problem I got, or some notices I missed?