How to use Rails 4 strong parameters with has_many :through association?

Question

I'm having trouble getting a has_many :through association working with Rails 4's strong parameters. I have a model called Checkout and I need to select a person from the Employee model in the new checkout form. Checkouts and Employees are associated through an Employment model.

I'm getting this error when I try to create a new checkout:

NoMethodError in CheckoutsController#create
undefined method `employee' for #<Checkout:0x007ff4f8d07f88>

It seems that there's something wrong with either my create action, my checkout parameters or my new checkout form. Here's the create action:

  def create    
    @user = current_user
    @checkout = @user.checkouts.build(checkout_params)

    respond_to do |format|
      if @checkout.save
        format.html { redirect_to @checkout, notice: 'Checkout was successfully created.' }
      else
        format.html { render action: 'new' }
      end
    end
  end

My checkout params:

def checkout_params
      params.require(:checkout).permit(:job, :employee_ids, :shift, :date, :hours, :sales, :tips, :owed, :collected, :notes)
end

My new checkout form:

<div class="field">
     <%= f.label :employee %><br>
     <%= f.collection_select(:employee_ids, Employee.all.collect, :id, :full_name, {:prompt => "Please select"} ) %>
</div>

But I can't figure out what has changed with Rails 4 and strong parameters. In Rails 3 this type of association and form worked for me using attr_accessible instead of strong_parameters.

Relevant Files

Full Trace of the error: https://gist.github.com/leemcalilly/0cb9e2b539f9e1925a3d

models/checkout.rb: https://gist.github.com/leemcalilly/012d6eae6b207beb147a

controllers/checkouts_controller.rb: https://gist.github.com/leemcalilly/a47466504b7783b31773

views/checkouts/_form.html.erb https://gist.github.com/leemcalilly/ce0b4049b23e3d431f55

models/employee.rb: https://gist.github.com/leemcalilly/46150bee3e6216fa29d1

controllers/employees_controller.rb: https://gist.github.com/leemcalilly/04f3acdac0c9a678bca8

models/employment.rb: https://gist.github.com/leemcalilly/6adad966dd48cb9d1b39

db/schema.rb: https://gist.github.com/leemcalilly/36be318c677bad75b211

Answer 1

Keep in mind that the name you give to your strong parameters (employees, employee_ids, etc.) is largely irrelevant because it depends on the name you choose to submit. Strong parameters work no "magic" based upon naming conventions.

The reason https://gist.github.com/leemcalilly/a71981da605187d46d96 is throwing an "Unpermitted parameter" error on 'employee_ids' is because it is expecting an array of scalar values, per https://github.com/rails/strong_parameters#nested-parameters, not just a scalar value.

# If instead of:
... "employee_ids" => "1" ...
# You had:
... "employee_ids" => ["1"]

Then your strong parameters would work, specifically:

... { :employee_ids => [] } ...

Because it is receiving an array of scalar values instead of just a scalar value.