How can I use accepts_nested_attributes_for to create 1 related object in a join table?

Question

I have a very simple design at present, created using Ruby 2.0 and the Rails 4 beta. I have users, and groups. A User can be a member of multiple groups, and a Group can have multiple users as members. I thus have a join model, UserGroup, which only contains user_id and group_id. As follows: ``` class User < ActiveRecord::Base has_many :user_groups, dependent: destroy has_many :groups, through: :user_groups accepts_nested_attributes_for :user_groups, allow_destroy: true end class Group < ActiveRecord::Base has_many :user_groups, dependent: destroy has_many :users, through: :user_groups end class UserGroup < ActiveRecord::Base belongs_to :user, inverse_of: :user_groups belongs_to :group, inverse_of: :user_groups validates :user_id, presence: true validates :group_id, presence: true end ``` I have found lots of stuff about accepts_nested_attributes_for which seems to show creating a single associated object on create/update. What I want, is a multi option select box where I can select m…

Accepted Answer

Turns out what I actually needed to do was fix my strong_parameters hash. It was causing my params[:user][:group_ids] to be rejected from the update method. I needed to tell strong_parameters it was an array of simple attributes, thusly:

```
def user_params
    params.require(:user).permit(:forename, :surname, :email, group_ids: [])
end
```

How can I use accepts_nested_attributes_for to create > 1 related object in a join table?

Solution

Problem