Why did Rails 5 changed “index” to “foreign key”?

Question

If you had this in Rails 4:

t.references :event, index: true

Now you could use foreign_key instead of index in Rails

Answer 1

foreign_key and index are completely different things (as you may judge from their names).

So nothing is being changed, you still can use two.

You can check out these docs for some more info on establishing associations in migrations.