Different Ways To Add A Foreign Key Reference

A foreign key reference creates a relationship between two tables that is guaranteed by a foreign key constraint.

This is a minimal example.

create_table :books
  t.references :author, foreign_key: true
end

The foreign_key: true is needed here, otherwise just the reference column is created without a backing constraint. When foreign_key is true, an index will be created for the column as well.

This is a maximal example.

create_table :books
  t.references :author, index: true, foreign_key: true, type: :uuid, null: false
end

It is explicit about the foreign key and index. It specifies a not null constraint. It declares the type as uuid assuming the authors table's primary key is of type uuid.

Here is an example with a custom column name.

create_table :books
  t.references :written_by, foreign_key: { to_table: :authors }
end

Here is adding a reference to an existing table.

def up
  add_reference :books, :author, index: true, foreign_key: true
end

There are more combinations of these, but I hope there is enough here to be able to iterate to a solution that works for you.