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.
In rails 4, you can simple use
create_join_table :table1s, :table2s
it is all.
Caution: you must offord table1, table2 with alphanumeric.
I like doing:
rails g migration CreateJoinedTable model1:references model2:references
. That way I get a migration that looks like this:
class CreateJoinedTable < ActiveRecord::Migration
def change
create_table :joined_tables do |t|
t.references :trip, index: true
t.references :category, index: true
end
add_foreign_key :joined_tables, :trips
add_foreign_key :joined_tables, :categories
end
end
I like having index on these columns because I'll often be doing lookups using these columns.
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