Rails nested form Unpermitted parameter - ruby-on-rails

Question

When i create or edit product, my merchant_product_detail will not be created or edit due to
Unpermitted parameter: merchant_id. But other like id and price are able to pass strong parameters, only merchant_id could not pass it. Kindly help me why my merchant_id is not permit in this case?
params return in binding.pry mode
"product"=>
{"name"=>"fewrgvers",
"description"=>"",
"product_categories_attributes"=>{"0"=>{"id"=>""}},
"merchant_product_details_attributes"=>
{"0"=>{"merchant_id"=>["2"], "id"=>"", "price"=>"123"}}
"
product_params return
Unpermitted parameter: merchant_id
=> {"name"=>"fewrgvers",
"description"=>"",
"merchant_product_details_attributes"=>{"0"=>{"id"=>"", "price"=>""}}
product.rb
has_many :merchant_product_details
accepts_nested_attributes_for :merchant_product_details, reject_if: proc { |attributes| attributes['merchant_id'].blank? }
has_many :merchants, through: :merchant_product_details
merchant.rb
has_many :merchant_product_details
has_many :products, through: :merchant_product_details
accepts_nested_attributes_for :merchant_product_details
merchant_product_detail.rb
belongs_to :product
belongs_to :merchant
product_controller.rb
def new
#product = Product.new
#product.merchant_product_details.build
end
def create
#product = Product.new(product_params)
respond_to do |format|
if #product.save
format.html { redirect_to root_path, notice: 'Product was successfully created.' }
else
format.html { render :new }
end
end
end
end
def update
respond_to do |format|
if #product.update_attributes(product_params)
format.html { redirect_to root_path, notice: 'Product was successfully updated.' }
else
format.html { render :edit }
end
end
end
params.require(:product).permit(:name, :description,
merchant_product_details_attributes: [:id, :merchant_id, :price]
_form.html.erb
<%= form_for(#product, :url => path, html: { class: "form-horizontal", role: "form" }) do |f| %>
<%= f.fields_for :merchant_product_details do |builder| %>
<div class="form-group row">
<%= builder.label :merchant_id, class: "col-md-2 control-label" %>
<div class="col-md-8">
<%= builder.select :merchant_id, Merchant.all.collect {|x| [x.name, x.id]}, {include_hidden: false} ,prompt: "Select something", multiple: true, class: "select2" %>
<%= builder.hidden_field :id %><br>
<%= builder.label :price %>
<%= builder.number_field :price %>
</div>
<% end %>

Answer 1

The problem is the multiple: true in the form field for merchant_id. This means that the param will be an array, as it could be multiple merchant ids.
If this is what you want then I would recommend changing the name to merchant_ids and allow an array like this:
params.require(:product).permit(:name, :description,
merchant_product_details_attributes: [:id, :price, merchant_ids: []])
Having a look at your model relations I think you only want to have one id though, in which case it should be enough to remove the multiple: true in the select.
<%= builder.select :merchant_id, Merchant.all.collect {|x| [x.name, x.id]}, {include_hidden: false}, prompt: "Select something", class: "select2" %>