Add A Foreign Key Reference To A Table

Foreign keys are a great way to maintain referential integrity within our data. We can add reference columns with foreign key constraints using the Rails migration DSL.

Here is how we include one as part of creating a table:

def up
  create_table :books do |t|
    # ... other columns

    t.references :author, index: true, foreign_key: true
  end
end

This will add a column, author_id, to the books table that references the authors table. It will have both a foreign key constraint and an index applied to it.

Here is how we do the same for an existing table:

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

As of Rails 5, this is a bit verbose as index: true happens by default. Though I'm always in favor of explicitness. If for whatever reason you don't want an index, you will have to specify index: false.