For Rails 5+

Initial Definition:

If you are defining your Post model table, you can set references, index and foreign_key in one line:

t.references :author, index: true, foreign_key: { to_table: :users }

Update Existing:

If you are adding references to an existing table, you can do this:

add_reference :posts, :author, foreign_key: { to_table: :users }

^{Note: The default value for index is true.}

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 migrationadd_reference :posts, :author, index: trueadd_foreign_key :posts, :users, column: :author_id# The modelbelongs_to :author, class_name: "User"

In rails 4, when using postgresql and the schema_plus gem you can just write

add_reference :posts, :author, references: :users

This will create a column author_id, which correctly refers to users(id).

And in your model, you write

belongs_to :author, class_name: "User"

Note, when creating a new table you can write it as follows:

create_table :things do |t|   t.belongs_to :author, references: :users end 

Note: the schema_plus gem in it's entirety is not compatible with rails 5+, but this functionality is offered by the gem schema_auto_foreign_keys (part of schema_plus) which is compatible with rails 5.