ERROR: delete on table violates foreign key constraint. Key id is still referenced from table (many)

From Rails v4.2 you can do this:

Create a migration to update the foreign keys

20160321165946_update_foreign_key.rb

class UpdateForeignKey < ActiveRecord::Migration
  def change
    # remove the old foreign_key
    remove_foreign_key :posts, :users

    # add the new foreign_key
    add_foreign_key :posts, :users, on_delete: :cascade
  end
end