Unevaluable formulas return nil (instead of throwing error)

Filter formula errors are shown in filter view

app/controllers/ingredients_controller.rb

@@ -196,13 +196,13 @@ class IngredientsController < ApplicationController
   end
 
   def prepare_ingredients
-    @ingredients = @project.ingredients.includes(:ref_unit, :source)
+    @ingredients, @formula_q = @project.ingredients.includes(:ref_unit, :source)
       .filter(@project, session[:filters])
   end
 
   def prepare_nutrients
     @quantities = @project.quantities.where(primary: true)
-    ingredients, requested_n, extra_n = @project.ingredients
+    ingredients, requested_n, extra_n, @formula_q = @project.ingredients
       .filter(@project, session[:filters], @quantities)
 
     @nutrients = {}

app/models/ingredient.rb

@@ -49,18 +49,20 @@ class Ingredient < ActiveRecord::Base
     end
 
     formula_q = if filters[:nutrients].present?
-      formula = Formula.new(project, filters[:nutrients])
-      if formula.valid?
-        project.quantities.new(name: '__internal_q', formula: filters[:nutrients])
-      end
-    end
+                  project.quantities.new(name: '__internal_q',
+                                         formula: filters[:nutrients],
+                                         domain: :diet)
+                end
+    apply_formula = formula_q.present? && formula_q.valid?
 
-    if !requested_q.empty? || formula_q.present?
-      result = self.compute_nutrients(requested_q, formula_q)
-      requested_q.present? ? result : result[0]
-    else
-      ingredients
-    end
+    result =
+      if !requested_q.empty? || apply_formula
+        computed = ingredients.compute_nutrients(requested_q, apply_formula && formula_q)
+        requested_q.present? ? computed : [computed[0]]
+      else
+        [ingredients]
+      end
+    result.push(formula_q)
   end
 
   def self.compute_nutrients(requested_q, filter_q = nil)

app/views/ingredients/_filters.html.erb

@@ -1,5 +1,7 @@
 <fieldset id="filters" class="collapsible">
   <legend onclick="toggleFieldset(this);"><%= l(:label_filter_plural) %></legend>
+  <%= error_messages_for @formula_q %>
+
   <%= form_tag url, id: 'filters_form', method: :get, remote: true do %>
   <table class="filter">
     <tr>

app/views/ingredients/_list.html.erb

@@ -1,4 +1,7 @@
 <% if @ingredients.any? { |i| i.persisted? } %>
+  <%= render :partial => 'ingredients/filters',
+    :locals => {:url => filter_project_ingredients_path(@project)} %>
+
   <table class="list">
     <thead>
       <tr>

app/views/ingredients/_list_nutrients.html.erb

@@ -1,4 +1,7 @@
 <% if @nutrients.any? %>
+  <%= render :partial => 'ingredients/filters',
+    :locals => {:url => filter_nutrients_project_ingredients_path(@project)} %>
+
   <%= render :partial => 'ingredients/options' %>
 
   <table class="nutrients list odd-even">

app/views/ingredients/index.html.erb

@@ -13,8 +13,6 @@
 <%= render :partial => 'ingredients/form' %>
 
 <h2><%= t ".heading" %></h2>
-<%= render :partial => 'ingredients/filters',
-  :locals => {:url => filter_project_ingredients_path(@project)} %>
 <div id='ingredients'>
   <%= render :partial => 'ingredients/list' %>
 </div>

app/views/ingredients/nutrients.html.erb

@@ -13,8 +13,6 @@
 <%= render :partial => 'ingredients/form' %>
 
 <h2><%= t ".heading" %></h2>
-<%= render :partial => 'ingredients/filters',
-  :locals => {:url => filter_nutrients_project_ingredients_path(@project)} %>
 <div id='nutrients'>
   <%= render :partial => 'ingredients/list_nutrients' %>
 </div>