Add create_join_table migration helper to create HABTM join tables

[rafaelfranca]

This helper was created as described here.

It can help to avoid issues like this #4653.

• Docs and CHANGELOG line included

[josevalim]

I like it. It will reduce the mistakes related to join tables a lot. If other core team members agree with this patch, we could even add a generator shortcut like: rails g migration join_table_users_and_labels (yes, I am a lazy bastard)

[vijaydev]

CreateJoinTableTest

[rafaelfranca]

💣 Fixed. Thanks

[jeremy]

Seems easy to forget that there's a special way to create join tables, though.

Could we solve the original problem by just populating timestamps on join tables, too?

[josevalim]

@jeremy I am also fine with populating timestamps on join tables, although it seems @jonleighton doesn't like the idea much.

[jeremy]

I'm not a fan of more complexity there either, but it feels worth being able to use a "stock" create_table as a join table.

Less for people to think about or look up in docs, and it's interchangeable with a has_many :through without migrating.