Validation failed: Stocks product must exist, Stocks location must exist - ruby-on-rails

Question

I have three models -> locations, products and stocks.
Stocks is a join table of locations and products.
When creating a new product i used fields_for to show locations and even though i got it to work, for some reason now it does not seem to work anymore and it gives me the above error.
<div class="input-field">
<%= f.label :product_name %>
<%= f.text_field :name, autofocus: true %>
</div>
<div class="input-field">
<%= f.label :price %>
<%= f.text_field :price, autofocus: true %>
</div>
<% if !#edit %>
<%= f.fields_for :stocks do |ff| %>
<div class="input-field margin-top x-4">
<%= ff.collection_select :location_id, Location.all, :id, :structured_location , {:prompt => "Please Select Locations for Product"}, {multiple: true} %>
<%= ff.label :locations %>
</div>
<div class="input-field">
<%= ff.label :quantity %>
<%= ff.text_field :quantity %>
</div>
<div class="input-field">
<%= ff.label :threshold_quantity %>
<%= ff.text_field :threshold_quantity %>
</div>
<% end %>
<% else %>
<%= collection_select :product, :location_ids, Location.all, :id, :structured_location , {:prompt => "Please Select Locations for Product"}, {multiple: true} %>
<% end %>
<div class="row margin-top x-4">
<div class="col s12 center-align">
<%= f.submit "#{current_page?(new_product_path) ? "Create Product" : "Update Product"}", class: "btn wave-effect pink darken-1 btn-large" %>
</div>
</div>
controller
class ProductsController < ApplicationController
helper_method :sort_column, :sort_direction
def index
#products = Product.order(sort_column + " " + sort_direction)
end
def new
#product = Product.new
#product.stocks.build
end
def create
#product = Product.new(product_params)
if #product.save!
flash[:notice] = "Successfully saved..."
redirect_to products_path
else
flash[:alert] = "Something went wrong, please check the values you entered"
redirect_to :back
end
end
private
def product_params
params.require(:product).permit(:name,:price, location_ids: [], stocks_attributes: [:id, :quantity, :threshold_quantity, location_id: []])
end
end
product model
class Product < ApplicationRecord
has_many :stocks, dependent: :destroy
has_many :locations, :through => :stocks
accepts_nested_attributes_for :stocks
end
parameters in rails console
Parameters: {"utf8"=>"✓",
"authenticity_token"=>"l1BFhrdyB2QMO5k3+60GNiPphFfF+DXDGPbUU3V2Op2aekObjgIe13k8uoedmDIEZgIeXPZUeS/0VxQXkKa1Uw==",
"product"=>{"name"=>"Soap", "price"=>"10", "location_ids"=>["", "1",
"2"], "stocks_attributes"=>{"0"=>{"quantity"=>"100",
"threshold_quantity"=>"100"}}}, "commit"=>"Create Product"}

Answer 1

After hours of searching i stumbled upon this post
BigBinary
it seems that Rails 5 made belongs_to relationship IDs required by default thats why i had validations failed and i couldn't find anything about it.
simply adding optional: true in my stock model worked!
class Stock < ApplicationRecord
belongs_to :location, optional: true
belongs_to :product, optional: true
accepts_nested_attributes_for :product, allow_destroy: true
accepts_nested_attributes_for :location, allow_destroy: true
end