How to change value in params array in Ruby on Rails?

Question

I have an instance where I am recording prices for water from vendors. My vendor model has :price. However, I want to give users the option to input a price for different volumes, and do the simple division for them rather than having them to do it. In other words, users should be able to input $1.99 per liter or $3.99 for a gallon and so on. To do this, I need a virtual attribute in my form for :unit, since I don't want to be storing units in the table. Everything works well, except that I cannot seem to update vendor_params[:price] before I update the record or create a new record. This seems like it should be a cake walk, but I Googled most of the day and can't figure out how to make it work.
Here is what I have:

Model:

class Vendor < ActiveRecord::Base
   attr_accessor :unit
...
end

Form:

<%= form_for(@vendor) do |f| %>
...
<div class="field">
    <%= f.label :price %><br>
    <%= f.text_field :price %>
    <%= select( "unit", "id", { "1 Liter" => "1", "Bottle (2 liters)" => "2", "Jerry Can  (20 liters)" => "20"}) %>
</div>
...
<% end %>

Controller:

...
def update 

vendor_params[:price] = vendor_params[:price].to_f/params[:unit][:id].to_f

respond_to do |format|
  if @vendor.update(vendor_params)
    format.html { redirect_to @vendor, notice: 'Vendor was successfully updated.' }
    format.json { head :no_content }
  else
    format.html { render action: 'edit' }
    format.json { render json: @vendor.errors, status: :unprocessable_entity }
  end
end
...
end

I know that vendor_params[:price].to_f/params[:unit][:id].to_f returns the correct value. I just can't seem to assign that value to vendor_params[:price] before I update the record. I also tried the following which throws an error:

@vendor_params[:price] = vendor_params[:price].to_f/params[:unit][:id].to_f

It seems like this should be trivial! I guess I could use form_tag instead of form_for, but that seems odd when updating the full record. (The edit form has all fields for all the object attributes.) Anywho, I'm open to ideas and suggestions.
Thanks!!

Community · Accepted Answer · 2017-05-23 10:26:48Z

2

If vendor_params is a strong_params method (which I'm assuming it is), it actually creates a new hash. So when you alter vendor_params... you're not actually changing your original params hash.

OK, why isn't vendor_params changing though... I dont care about params? WELL, vendor_params still points the original params hash assuming it looks something like:

def vendor_params
  params.require(:vendor).permit(:price)
end

I think the link below is a similar issue and may present a useful solution. Hope I understood your problem correctly!

Modify ruby hash in place( rails strong params)

edited May 23, 2017 at 10:26

CommunityBot

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answered May 1, 2014 at 5:08

rsahai91

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1 Comment

isergio Over a year ago

Thanks @rsahai91! Your explanation and link gave me an idea how to fix it. I basically created an instance variable like so: vp = vendor_params and passed the form hash vp[:price] = vendor_params[:price].to_f/params[:unit][:id].to_f then just updated with the instance variable: @vendor.update(vp)

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