add_reference :books, :author
add_column :books, :author_id, :integer
Here add references will create user_id column and add column is also creating user_id column in books table. What is the difference between them. What is the advantage of using references instead of column?
#add_column is meant for adding a column like the name suggests.
#add_reference is meant as a shortcut for creating a column, index and foreign key at the same time.
In your example the only difference is the index on the column that will be created by #add_reference that defaults to true.
add_reference :books, :author
# equals
add_column :books, :author_id, :integer
add_index :books, :author_id
But if you would take the following line:
add_reference :books, :author, foreign_key: true
It would also create a foreign key constraint.
Furthermore if you would like to have every author be able to publish only one book you can set the unique constraint through #add_reference by doing the following:
add_reference :books, :author, null: false, index: {unique: true}, foreign_key: true
This requires every book to have an author and restraints the authors to have a maximum of one book.
The same can be done using #add_column by doing the following:
add_column :books, :author_id, :integer, null: false
add_index :books, :author_id, unique: true
add_foreign_key :books, :authors
Both will generate the same columns when you run the migration.
The first command adds a belongs_to :author relationship in your Book model whereas the second does not. When this relationship is specified, ActiveRecord will assume that the foreign key is kept in the author_id column and it will use a model named Author to instantiate the specific author.
The first command also adds an index on the new author_id column.