How to save selected values from one table column to another table column

Question

I am new to Rails, and trying to learn. In my simple form I have created a drop down select with generated data from a table called professions. This part is working fine, and I can select multiple values. I'm using a mysql database.

When I click the submit button, I need it to save the chosen values to a column called my_professions in another table called users. I can’t figure out how to do this. I get this error

My form

<%= simple_form_for @user, url: wizard_path, method: :put do |f| %>
<%= collection_select(:f, :professions_id, Profession.where.not(name: nil), :id, :name, {:multiple => true}, {:class=>'js-example-basic-multiple', :id=>'jsmultipleddd'}) %>
<%= f.submit "Save", :class => 'btn blue'  %> 
<% end %>

I have tried to add this to the user model

user.rb

class User < ApplicationRecord

has_many :professions
accepts_nested_attributes_for :professions

serialize :my_professions, Array
end

And this to the profession model

profession.rb

class Profession < ApplicationRecord
belongs_to :user
end

my params look like this

registration_steps_controller.rb

def user_params

  params.require(:user).permit(:gender,:practitioner_website, :public_health_insurance, clinic_images: [], professions: [])

end

application_controller.rb

def configure_permitted_parameters
  devise_parameter_sanitizer.permit(:sign_up, keys: [:gender, :practitioner_website, :public_health_insurance, clinic_images: [], professions: []])
  devise_parameter_sanitizer.permit(:account_update, keys: [:gender, :practitioner_website, :public_health_insurance, clinic_images: [], professions: []]) 
end

Answer 1

Start by getting rid of accepts_nested_attributes_for :professions. You don't need nested attributes for this.

Then get rid of serialize :my_professions, Array. Serialize is a legacy hack to store complex data in string columns. You don't need or want this (ever), since associations should be stored in join tables in ActiveRecord - not array columns. That's how AR was designed to work and that's how relational databases where designed to work.

Instead what you want is a join model. Which you can generate with:

rails g model user_profession user:belongs_to profession:belongs_to

Run the migration. You then setup the associations between users and professions:

class User < ApplicationRecord
  # ...
  has_many :user_professions
  has_many :professions, through: :user_professions
end

class Profession < ApplicationRecord
  # ...
  has_many :user_professions
  has_many :users, through: :user_professions
end

Now this lets us associate users with professions by passing profession_ids.

In a normal Rails form you would create the input with:

<%= f.collection_select :profession_ids, Profession.all, :id, :name, multiple: true ... %>

In SimpleForm use the association helper:

<%= f.association :professions, ... %>

Then whitelist the correct param:

def user_params
  # don't jam this into one super long unreadable line
  params.require(:user)
        .permit(
          :gender, :practitioner_website, :public_health_insurance, 
          clinic_images: [], profession_ids: []
        )
end