Consider adding ability to specify additional join conditions on a @JoinTable / @JoinColumn

Per discussion with beberlei and romanb in #doctrine-dev yesterday, opening this ticket as a "feature request" to support migrating legacy schemas with a special many-to-many mapping to Doctrine.

Consider the following schema:

CREATE TABLE categories (
    category_id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
    content_type ENUM('posts', 'videos'),
    /* ... */
    PRIMARY KEY (category_id)
) ENGINE=InnoDB;

CREATE TABLE content_category_association (
    content_id BIGINT UNSIGNED NOT NULL,
    category_id BIGINT UNSIGNED NOT NULL,
    content_type ENUM('posts', 'videos'),
    PRIMARY KEY (content_id, category_id, content_type),
    FOREIGN KEY (category_id, content_type) REFERENCES categories(category_id, content_type) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB;

CREATE TABLE posts (
    post_id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
    /* ... */
    PRIMARY KEY (post_id)
) ENGINE=InnoDB;

CREATE TABLE videos (
    video_id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
    /* ... */
) ENGINE=InnoDB;

There is a Many-To-Many relationship between each of the posts and videos table (via the content_category_association table) to the categories table. The difference from a standard many-to-many relationship is there is an extra column in the association table (content_type) which must be included in the join condition to return correct results. Since both the videos and posts table have their own autonumber primary keys, a join against the association table must include an extra condition (i.e. INNER JOIN ... ON ... AND content_category_association.content_type = 'posts').

Perhaps you could allow passing of additional properties to @JoinTable / joinColumns to specify the additional join condition .. i.e.:

/** @Entity */
class Video
{
  /**
   * @ManyToMany(targetEntity="Category")
   * @JoinTable(name="content_category_association",
   *      joinColumns={@JoinColumn(name="content_id", referencedColumnName="video_id")},
   *      inverseJoinColumns={@JoinColumn(name="category_id", referencedColumnName="video_id")},
   *      extraJoinTerms={@JoinTerm(content_type="video")}
   *      )
   */
  private $categories;

  // ...
}

/** @Entity */
class Category
{
    // ...
}

Certainly this schema is not ideal from a pure OO perspective. Class inheritance with a discriminator column may have been a better way to do this, thereby allowing a globally unique "content_id" for all types of content, negating the need for the extra column in the association table. However, it would nonetheless be helpful to have this additional capability within Doctrine to avoid having to re-factor such a legacy schema.