Generating a model with many to many in Ruby on Rails

Question

Is there a way to generate a Rails model with a many to many relationship predefined? I know how to add it to the Active Record after the fact but it would be nice to have it defined in the DB migration and the Active Record model right off the bat.

score 22 · Answer 1 · edited May 16 '19 at 06:26

22

Remember that you do not want an id for the join table, so make sure to add :id => false |t|

create_table assemblies_parts, :id => false do |t|
  t.integer :assembly_id
  t.integer :part_id
end

If you use rails

rails generate model Assemblies_parts assembly:references part:references

you will have two indexes, but what you want is

# Add table index
add_index :assemblies_parts, [:assembly_id, :part_id], :unique => true

UPDATE

For Rails 5 use create_join_table instead to create that (id-less) table.

edited May 16 '19 at 06:26

Felix

4,510
2
31
46

answered Mar 22 '12 at 15:35

Sully

14,672
5
54
79

What is the benefit of using create_join_table? As the "old" syntax still works fine and is imho not less comprehensible. – Felix May 15 '19 at 14:12
1

@Felix, possibly to avoid adding the odd `, :id => false` – Sully May 15 '19 at 23:33
I see. Makes sense. – Felix May 16 '19 at 06:27

score 3 · Answer 2 · answered Mar 16 '11 at 16:58

3

You can use this reference from the Rails Guides.Here is the link. Also you will need to manually create the join table for those models using a migration.

e.g

    create_table :assemblies_parts, :force => true do |t|
      t.integer :assembly_id
      t.integer :part_id
    end

answered Mar 16 '11 at 16:58

Shiv

8,362
5
29
32

Thanks for the response, I have read that guide. Like I mentioned to another responder, I was just wondering if the generate model script could handle it in one step, and it looks like it can't so that's fine...would've been nice though. ;p – Kevin Mar 16 '11 at 18:03
yeah I dont think it can be done automatically. If I discover a short cut I will post it here :). – Shiv Mar 16 '11 at 21:39

score 1 · Answer 3 · edited May 23 '17 at 10:31

1

Please look at this question first: Creating a many-to-many relationship in Rails 3.

In addition, I would recommend next book "Ruby on Rails 3 Tutorial: Learn Rails by Example" for a better understanding of ActiveRecord relations.

edited May 23 '17 at 10:31

Community

1
1

answered Mar 16 '11 at 07:15

akrisanov

3,212
6
33
56

Thanks for the response, I have read the guide that post links to. I was just wondering if the generate model script could handle it in one step, looks like no. – Kevin Mar 16 '11 at 18:00

Generating a model with many to many in Ruby on Rails

3 Answers3

Linked