Quick Tip: has_many :through => checkboxes

It’s really easy to create a many-to-many relationship that can be assigned through checkboxes. Check it out!

Let’s say you have Users and Groups. A User can belong to a Group and a Group can have many Users – we call this a Membership, like so (migrations omitted for brevity):


class User < ActiveRecord::Base
  has_many :memberships, :dependent => :destroy
  has_many :groups, :through => :memberships


class Group < ActiveRecord::Base
  has_many :memberships, :dependent => :destroy
  has_many :users, :through => :memberships


class Membership < ActiveRecord::Base
  belongs_to :group
  belongs_to :user

We can now assign groups to members in a relatively easy manner with no extra work needed in the models. Behold!


<h1>User <%= @user.id -%></h1>

<h2>Group Memberships</h2>
<% form_for @user do -%>
  <% Group.all.each do |group| -%>
      <%= check_box_tag :group_ids, group.id, @user.groups.include?(group), :name => 'user[group_ids][]' -%>
      <%= label_tag :group_ids, group.id -%>
  <% end -%>
  <%= submit_tag -%>
<% end -%>

Errr… something like that. Anyway, the important thing to note is the use of group_ids. The values will get submitted as group_ids, a member of the User. Where did that come from? We don’t have an attribute or method on the model for it, so where’d it come from? Well, seems that it is auto-generated for you to allow something like I just showed.

When this form is submitted, any checked Groups will be associated through Memberships to the User by way of the magic *_ids= method. Should work the other way too with user_ids checkboxes on a group. No extra code needed. Awesome, right?

Bonus: If you uncheck all the checkboxes, then nothing gets posted, doh! So make sure to merge a default value with your parameters like this to ensure the *_ids= method gets called:


@user.attributes = {'group_ids' => []}.merge(params[:user] || {})

Super Bonus: When you’re defaulting the group_ids in the controller make sure to use the key as a string, not a symbol. Or if you do use a symbol then make it a Hash with_indifferent_access.

Super Monkey Ball: A monkey encased in a ball who collects bananas.

Posted August 26th, 2008 at 5:29 am in Ruby on Rails | Permalink


  1. Archie:

    Hi there.
    Thanks for the quick tip. As a newbie I have been struggling with above and think your solution to very simple compared to other examples posted elsewhere. Just one question: Where would you actually place the “merge(params[:user]”to make sure the *_ids= method gets called? I was thinking the update method but I am not sure. Perhaps it would be helpful to put inthis into context.
    Much appreciated.

  2. Mike:

    Thanks for sharing! You saved my day!
    No more messing around with model callbacks to do the job!

  3. @bpaul:

    Very nice, I love it when you can get Rails to do most of the work! Now if someone updated the check_box_tag to work nicely with arrays it would be even simpler…

  4. rcrogers:

    Word of warning — when you use the collection_ids method, Rails fails to call before_destroy and after_destroy on the join model, don't ask me why. I wasn't the only one to notice: http://dev.rubyonrails.org/ticket/7743

  5. Fer:

    Excelent! Very Usefull!

    greetings from Argentina

  6. nando:

    Thanks, it's been very useful for me too.

    I'd like to point something. In the HTML generated we have diferent checkbox elements with the same id. Also the label's for attribute has the same value for all of them, losing their purpose.

  7. meme:

    Awesome and thans

    As for Archie,

    You have to place that in "new" and "edit" controller

  8. cflannagan:

    re: Meme's comment

    Actually I think Archie is correct about putting it in "update" action.

    I tried putting it in "edit" and all it does is empty out the checkboxes upon reaching the edit view, leading me to think that line is meant to be processed AFTER the edit data is submitted (which makes more sense when I read that line).

    I moved it into "update" action and everything is behaving correctly now. I had no need to modify either "create" or "new" actions.

  9. Hesham:

    Thank you, you saved me hours of research

  10. Bruno Freitas:

    Great content.


  11. Charles Max Wood:

    Quick note on the labels. If you want to get the labels to work, set :id => "group_check_#{group.id}" on your checkbox and label_tag "group_check_#{group.id}", group.name for your lables.

  12. MrShoop:

    Thanks for this great post; it really helped me in getting this set up. I'm not sure I would have ever figured out how it worked without your description (I would have done a lot of extra manual handling).

  13. Efrén Fuentes:

    You need put this:

    attr_accesible :group_ids

    On user.rb to avoid WARNING: Can't mass-assign protected attributes: group_ids

  14. Jason:

    Thank you for sharing this tip. Nice solution. Saved me a lot of time.

  15. trbartel:

    Thanks, I'm much closer…But, what if your user and your group both belonged an overall Union model…say Union has many users and Union has many groups…Right now I'm getting all the checkboxes and groups for the whole app, instead of just one Union?

  16. Dmitry:

    Great tutorial!

  17. Nicolas Baptista:

    Hey man, great tip!

    Thank you.

  18. yaniv:

    How do I get the 'update' RSpec to pass using this technique?

  19. Evgeny:

    Thank you sir, this helped me immensely, even all these years later.

  20. nachopro:

    Muchas gracias por el aporte! Saludos desde Argentina

  21. Eric Goldberg:

    Or you could just do:

    params[:user][:group_ids] ||= []

    In your controller's update method, before calling update_attributes.

  22. David Mauricio:

    And how to validate for atleast 1 memership?
    I've tried:
    validates :memership, :length => { :minimum => 1} #must have atleast 1 record in the HABTM relation

    But it does return activerecord validation errors while checking many memerships or none

    Nice post!

  23. Vicky D:

    This post helped me a lot. Thank you so much.
    It would be helpful to know more about such scenario for ex. if after adding groups to users, we want to go back and add some more groups i.e. the page shows only those groups which have not yet been added and then can add them.

    This post saved me a lot of time. I wonder how would I have figured it out. Thanks a ton

  24. Giang Nguyen:

    Cool post. 2 things:

    1) <%= check_box_tag :group_ids, group.id, @user.groups.include?(group), :name => 'user[group_ids][]' -%>

    should be <%= check_box_tag "group_id_#{group.id}", …. -%>

    so that we don't have the same id for all checkboxes.

    2) Add this hidden field to the form then unchecking all boxes will work just fine without any further things to do in controller

    <%= hidden_field_tag :group_ids, nil, name: "user[group_ids][]" -%>


  25. Milind:

    Thanks…..it helped me to create groups has many users and users has many groups ….keep up the good work.

  26. Greg Charles:

    Editing these records seems to be problematic. If we edit a User and change the Memberships and then save, the group_ids method is called by the controller while applying the parameters, and this immediately updates the database. If the User record is then invalid and the save fails, the User fields are rolled back, but the changes to the Memberships persist. The solution to this seems to be wrapping the apply parameters step and and save! step inside a transaction, except that Rails considers transactions in the controller to be a bad practice. What's the right way to handle this?

  27. @ralph_metel:

    THANK YOU mate! Was evaluating 2 days long, before i read your comment to make this attribute accessible! 😉

  28. Keylor:

    Thank you. Thank you one thousand times. I was at the edge of crying on this one, but I finally found the solution on your blog.

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