How to add an instance of a model to an instance of another in Rails

Question

The model Patient belongs_to FeedList, which has_many patients. I'm attempting, upon the creation of a new FeedList I want to add all patients that are currently in the DB to the FeedList. I'm currently attempting to do it within create of feed_lists_controller.rb.

def create
    @feed_list = FeedList.new(feed_list_params)

    Patients.all.each do |p|
      @feed_list.patients << p
    end

    respond_to do |format|
      if @feed_list.save
        format.html { redirect_to @feed_list, notice: 'Feed list was successfully created.' }
        format.json { render :show, status: :created, location: @feed_list }
      else
        format.html { render :new }
        format.json { render json: @feed_list.errors, status: :unprocessable_entity }
      end
    end
  end

however, it doesn't seem to be registering when I create a new FeedList

[8] pry(main)> FeedList.create
=> #<FeedList:0xbaf7bdd0
 id: 3,
 created_at: Sun, 29 Nov 2015 01:11:54 UTC +00:00,
 updated_at: Sun, 29 Nov 2015 01:11:54 UTC +00:00,
 date: nil,
 integer: nil>
[9] pry(main)> FeedList.last.patients
=> #<Patient::ActiveRecord_Associations_CollectionProxy:0x-2284f570>

FeedList.rb:

class FeedList < ActiveRecord::Base
    has_many :patients

after_create :add_patients
  private
    def add_patients
      ::Patients.all.each do |p|
        self.patients << p
      end
    end

end

Am I on the right track?

The create action isn't a callback that gets invoked every time you create an instance of a model, it's a controller action that runs whenever a route is visited that is tied to it. At least, that's how it's usually set up. Where is this create function? — Jason
– Jason, Commented Nov 28, 2015 at 21:29
I'm guessing that you're editing someone else's code, right? That's a normal controller create action that you're editing. That action will be called whenever a client (or you) visits the route that is associated with it. Run rake routes from a terminal window to see what URI Pattern is associated with the Controller#Action feed_lists#create. If you visit that page and create a new FeedList using the form there, it will run your code. — Jason
– Jason, Commented Nov 28, 2015 at 21:35

Community · Accepted Answer · 2017-05-23 12:23:37Z

2

Try using a callback in your FeedList model file:

after_create :add_patients

  private
    def add_patients
      Patient.all.each do |p|
        self.patients << p
      end
    end

As Jason mentioned above "create action [that is in your controller] isn't a callback that gets invoked every time you create an instance of a model". See What is a callback function? for information on what is a callback. The after_create callback essentially says to invoke add_patients right after you call FeedList.create

edited May 23, 2017 at 12:23

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answered Nov 28, 2015 at 21:34

Kent Shikama

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12 Comments

Vincent Rodomista Over a year ago

thank you, i'll try that now. Can you explain to a novice what a "callback" is and why you have to putafter_create :add_patients at the top?

Jason Over a year ago

A callback is a generic programming term for a function that runs when your code finishes doing something, like reading a file, opening a video stream, or creating a database record.

Vincent Rodomista Over a year ago

Now I'm getting NameError: uninitialized constant FeedList::Patients How do I initialize it within the FeedList model?

Kent Shikama Over a year ago

Hmm what does your FeedList model class look like? A quick fix might be ::Patients.all.each but I would need to see the file.

Kent Shikama Over a year ago

You might want to open a new question. You would probably want to give which line the error comes from and the surrounding lines. It also might be worth taking a look at api.rubyonrails.org/classes/ActiveModel/…

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