Doctrine inheritance performance - symfony

Question

In my app, I have a few entities that share some common properties. Example entities:
Image
Video
Event
ForumPost
I want these classes to share some common functions, such as:
Have a list of comments, as well as keep counts of comments
Voting
List of subscribers and subscriber counts
View counts
etc..
I really would not want duplicate these functions for each of the entities above. So... I've been reading about Doctrine inheritance mapping and I've come up with an idea that "Class Table Inheritance" is the most elegant way to solve this. The plan was as follows:
Create a parent Post entity
/**
* #ORM\Entity
* #ORM\InheritanceType("JOINED")
* #ORM\DiscriminatorColumn(name="type", type="string")
*/
class Post
{
protected $likeCount;
protected $subscriberCount;
protected $subscribers;
// ...
}
Have many-to-one relations of likes, comments, votes, etc to Post entity:
class PostLike
{
/**
* #ORM\ManyToOne(targetEntity="Post")
* #ORM\JoinColumn(name="post_id", referencedColumnName="id", nullable=false)
*/
protected $post;
}
Have the Image, Video, etc entity extend Post:
class Video extends Post
{
// Post properties become available here
}
All worked like charm until I looked at the actual queries Doctrine is executing. The problem is: whenever I select a PostLike, doctrine will eager-load Post entity as well, which will in turn join all the child tables as well. So for example, a simple PostLike remove operation triggers a join on 4 tables and will actually join even more tables as my hierarchy grows.
If I have 10 entities extending Post, that would make a join on 10 tables. I am not a fan of premature optimization, but this just doesn't sound like a good plan.
In Doctrine documentation, this behavior is actually mentioned at "6.3.2. Performance impact".
Finally, my question: What are my other options to create reusable tables with doctrine?
I was thinking to do a One-To-One mapping (Post entity being a property of the Video, as well as the Image, etc), but I've read in a few blogs that One-to-One should be avoided with doctrine also for performance reasons.
thanks!
Edit and ansewer to Raphaël Malié regarding Mapped Superclasses:
I am aware that I can use Mapped Superclass and Trait features to avoid code duplication. However, these features allow me to reuse the code but don't allow me to reuse sql tables. Here is an example:
Say I want Image, Video and Event entities to have comments. I can create one Mapped Superclass AbstractComment and then extend it in ImageComment, VideoComment, EventComment. Next, I want to add comment votes. Since I can't reuse same sql table, I will need to create ImageCommentVote, VideoCommentVote, EventCommentVote, etc. Then I want users to be able to report abusive comments. There goes again: ImageCommentReport, VideoCommentReport, EventCommentReport. You get the idea. Every feature that I want to add to comment will need a separate table for each entity like image, video, etc. Lots of tables.
The reasons why I prefer to use centralized approach instead of using traits with many tables:
Easier administration. I can easily search/edit/delete comments of the entire app by sending a query to one table.
The same controller can handle actions for multiple entities, no need to create one abstract and many concrete controllers
Showing user notifications about new comments/likes etc is MUCH easier when actions are centralized.
I am also aware of the drawbacks of such system, but I can live with them for now. My question is how to get Doctrine to do what I want without actually joining everything when not necessary.

Answer 1

In my opinion you are doing wrong. Your entities have nothing in common, except some functionalities, so there is no reason to use single table or class table inheritance.
You should use Traits: http://php.net/manual/en/language.oop5.traits.php
It works with Doctrine. You define one or several traits with some properties / methods with annotations, and then you import these traits in your entities.
You can also use a Mapped Superclass : http://doctrine-orm.readthedocs.org/projects/doctrine-orm/en/latest/reference/inheritance-mapping.html#mapped-superclasses which is quite close to your example, but with a different entity annotation.

Answer 2

After some thought, your decision really depends on the size of your database. If it is big the performance hit of the Class Tabel approach might be unacceptable. But if your tables are small, the performance hit is minor and you can use all the advantages of Class Table Inheritance, which you discussed yourself in your post.