Custom Active Admin form inputs for has_and_belongs_to_many relationship

Question

I have a very simple model ``` class Lifestyle < ActiveRecord::Base attr_accessible :name has_and_belongs_to_many :profiles end ``` that has a `has_and_belongs_to_many` relationship with `Profile` ``` class Profile < ActiveRecord::Base attr_accessible ... belongs_to :occupation has_and_belongs_to_many :lifestyles accepts_nested_attributes_for :lifestyles end ``` I want to use ActiveAdmin to edit the Profile object, but also assign Lifestyles to a profile. It should be similar to dealing with `belongs_to :occupation`, as this is sorted out automatically by ActiveAdmin to a dropbox with the options pre-filled with available occupations. I've tried to use the `has_many` form builder method, but that only got me to show a form to type in the name of the Lifestyle and on submission, it returned an error. ``` f.object.lifestyles.build f.has_many :lifestyles do |l| l.input :name end ``` Error I get: ``` Can't mass-assign protected attributes: lifestyles_attributes ``` The perfect way for me…

Accepted Answer

After some research, I am ready to answer my own question.

First, I have to say thanks to @Lichtamberg for suggesting the fix. However, that only complicates things (also regarding security, though not an issue in this case), and doesn't help me reach my ideal solution.

Digging more, I found out that this is a very common scenario in Rails, and it's actually explained in Ryan Bates' screencast no #17.

Therefore, in Rails, if you have a has_and_belongs_to_many (short form HABTM) association, you can easily set the ids of the other associated object through this method:

```
profile.lifestyle_ids = [1,2]
```

And this obviously works for forms if you've set the attr_accessible for lifestyle_ids:

```
class Profile < ActiveRecord::Base
  attr_accessible :lifestyle_ids
end
```

In ActiveAdmin, because it uses Formtastic, you can use this method to output the correct fields (in this case checkboxes):

```
f.input :lifestyles, as: :check_boxes, collection: Lifestyle.all
```

Also, I have simplified my form view so it's now merely this:

```
  form do |f|
    f.inputs # Include the default inputs
    f.inputs "Lifestlyes" do # Make a panel that holds inputs for lifestyles
      f.input :lifestyles, as: :check_boxes, collection: Lifestyle.all # Use formtastic to output my collection of checkboxes
    end
    f.actions # Include the default actions
  end
```

Ok, now this rendered perfectly in the view, but if I try and submit my changes, it gives me this database error:

```
PG::Error: ERROR:  null value in column "created_at" violates not-null constraint
: INSERT INTO "lifestyles_profiles" ("profile_id", "lifestyle_id") VALUES (2, 1) RETURNING "id"
```

I found out that this is due to the fact that Rails 3.2 doesn't automatically update the timestamps for a HABTM association table (because they are extra attributes, and Rails only handles the `_id` attributes.

There are 2 solutions to fix this:

- Either convert the association into a hm:t (`has_many, :through =>`)

- Or remove the timestamps from the table

I'm going to go for 2) because I will never need the timestamps or any extra attributes.

I hope this helps other people having the same problems.

Edit: @cdesrosiers was closest to the solution but I already wrote this answer before I read his. Anyway, this is great nevertheless. I'm learning a lot.

Custom Active Admin form inputs for has_and_belongs_to_many relationship

Solution

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