Foreign key for named relations

named_reference.md

In Rails 4.2+ you can also set foreign keys in the db as well, which is a great idea.

For simple associations this can be done also on t.references adding foreign_key: true, but in this case you'll need two lines.

    # The migration
    add_reference :posts, :author, references: :users, index: true
    add_foreign_key :posts, :users, column: :author_id

    # The model
    belongs_to :author, class_name: "User"

from this SO answer