Doctrine ManyToMany distinction field - possible?

Doctrine ManyToMany distinction field - possible?

I'm trying to achieve a subscription model that can be applied to multiple
entities using a single table/class with Doctrine 2. See explanation by
example below.
Schema (yml):
User:
type: entity
table: users
id: int
name: string
Subscription:
type: entity
table: subscriptions
id: int
object_type: string
object_id: int
user_id: int
Feature:
type: entity
table: features
id: int
name: string
manyToMany:
subscribers:
targetEntity: User
joinTable:
name: subscriptions
joinColumns:
object_id:
referencedColumnName: id
Issue:
type: entity
table: issues
id: int
subject: string
manyToMany:
subscribers:
targetEntity: User
joinTable:
name: subscriptions
joinColumns:
object_id:
referencedColumnName: id
The table data would look something like this:
users:
| id | name |
| 1 | John |
| 2 | Joe |
features:
| id | name |
| 1 | Feature A |
| 2 | Feature B |
issues:
| id | subject |
| 1 | Issue 1 |
| 2 | Issue 2 |
subscriptions:
| id | object_type | object_id | user_id
| 1 | feature | 1 | 1 <- John is subscribed to Feature A
| 2 | issue | 1 | 1 <- John is subscribed to Issue 1
What I'd expected to have is an additional 'distinction' field that I can
have in the model's manyToMany relation for example:
manyToMany:
subscribers:
targetEntity: User
joinTable:
name: subscriptions
joinColumns:
object_id:
referencedColumnName: id
object_type:
value: feature
I know that the latter soultion doesn't exist in doctrine, however I'd be
curious how would you solve this situation?
The idea would be to dynamically extend this subscription "trait" to other
entities as well (eg. Project, Team, etc)
Do I have to introduce separate tables for all the subscriptions like
feature_subscribers and issue_subscribers.. and so on, or is there a more
elegant way?