Added abillity to use metacolumn as indexBy

[goetas]

Added ability to use meta column as indexBy. Useful if association entities is widely used.
Replace #204 PR

[doctrinebot]

Hello,

thank you for positing this Pull Request. I have automatically opened an issue on our Jira Bug Tracker for you with the details of this Pull-Request. See the Link:

http://doctrine-project.org/jira/browse/DDC-2384

[Ocramius]

Please fix CS - missing newline before this line

[Ocramius]

@goetas except for CS, this looks good to me

[beberlei]

@goetas in the testcase the indention is weird, and can you add a functional test as well? There is a testcsae which has lots of @group DDC-117 annotations, you can add it there because its related to the assocation identifier thing.

[goetas]

Added more tests

[webdevilopers]

Sorry for commenting on this closed PR. But a lot of people ask me about this feature recently.
And it looks like having to use the column names of foreign keys has never been documented:

• http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/tutorials/working-with-indexed-associations.html

Should an example be added to the docs?

    /**
     * @OneToMany(targetEntity="Stock", mappedBy="market", indexBy="market_id")
     * @var Stock[]
     */
    private $stocks;