Specifying column name in a "references" migration

Question

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

# The model
belongs_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.

Do it manually:

add_column :post, :author_id, :integer

but now, when you create the belongs_to statement, you will have to modify it, so now you have to call

def post
    belongs_to :user, :foreign_key => 'author_id'
end

If you aren't using a foreign key, then it doesn't matter what the actual table name of the other table is.

add_reference :posts, :author

As of Rails 5, if you're using a foreign key, you can specify the name of the other table in the foreign key options. (see https://github.com/rails/rails/issues/21563 for discussion)

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

Prior to Rails 5, you should add the foreign key as a separate step:

add_foreign_key :posts, :users, column: :author_id