Rails migration for has_and_belongs_to_many join table

How do I do a script/generate migration to create a join table for a has_and_belongs_to_many relationship?

The application runs on Rails 2.3.2, but I also have Rails 3.0.3 installed.

Where:

class Teacher < ActiveRecord::Base
  has_and_belongs_to_many :students
end

and

class Student < ActiveRecord::Base
  has_and_belongs_to_many :teachers
end

for rails 4:

rails generate migration CreateJoinTableStudentTeacher student teacher

for rails 3:

rails generate migration students_teachers student_id:integer teacher_id:integer

for rails < 3

script/generate migration students_teachers student_id:integer teacher_id:integer

(note the table name lists both join tables in alphabetical order)

and then for rails 3 and below only, you need to edit your generated migration so an id field is not created:

create_table :students_teachers, :id => false do |t|

A has_and_belongs_to_many table must match this format. I'm assuming the two models to be joined by has_and_belongs_to_many are already in the DB : apples and oranges:

create_table :apples_oranges, :id => false do |t|
  t.references :apple, :null => false
  t.references :orange, :null => false
end

# Adding the index can massively speed up join tables. Don't use the
# unique if you allow duplicates.
add_index(:apples_oranges, [:apple_id, :orange_id], :unique => true)

If you use the :unique => true on the index, then you should (in rails3) pass :uniq => true to has_and_belongs_to_many.

More information: Rails Docs

UPDATED 2010-12-13 I've updated it to remove the id and timestamps... Basically MattDiPasquale and nunopolonia are correct: There must not be an id and there must not be timestamps or rails won't allow has_and_belongs_to_many to work.

You should name the table the names of 2 models you want to connect by alphabetical order and put the two model id's in the table. Then connect each model to each other creating the associations in the model.

Here's an example:

# in migration
def self.up
  create_table 'categories_products', :id => false do |t|
    t.column :category_id, :integer
    t.column :product_id, :integer
  end
end

# models/product.rb
has_and_belongs_to_many :categories

# models/category.rb
has_and_belongs_to_many :products

But this is not very flexible and you should think about using has_many :through

The top answer shows a composite index that I don't believe will be used to lookup apples from oranges.

create_table :apples_oranges, :id => false do |t|
  t.references :apple, :null => false
  t.references :orange, :null => false
end

# Adding the index can massively speed up join tables.
# This enforces uniqueness and speeds up apple->oranges lookups.
add_index(:apples_oranges, [:apple_id, :orange_id], :unique => true)
# This speeds up orange->apple lookups
add_index(:apples_oranges, :orange_id)

I did find the answer this is based on by 'The Doctor What' useful and the discussion certainly so too.