Form including check_box_tag -- params not updating a has-many-through relationship

I'm trying to implement a check_box_tag to stock a Fridge model object with Ingredients; the two models are linked via a has-many-through relationship (FridgeIngredient model). It's worth noting that each Fridge also belongs to a User, which I worry might be causing the issue here in some manner (as this is the only difference I can see from where this code worked for me previously).

I've built a successful list of checkboxes for a has-many-through relationship previously, though it won't work on this occasion. The following form code (adapted for these models) is what worked elsewhere, but now fails to update the Fridge:

<%= form_for(@fridge) do |f| %>

    <!-- a debug column for testing -->
    <%= f.label :debug %>
    <%= f.text_field :debug

    <!-- the checkboxes -->
    <h2>Add ingredients to your Fridge</h2>
      <div>
          <% hidden_field_tag "fridge[ingredient_ids][]", nil %>
          <% Ingredient.all.each do |ingredient| %>
            <%= check_box_tag "fridge[ingredient_ids][]", ingredient.id, 
                @fridge.ingredient_ids.include?(ingredient.id), id: dom_id(ingredient) %>
            <%= label_tag dom_id(ingredient), ingredient.name %><br>
          <% end %>
      </div>
      <%= f.submit "Update" %>
<% end %>

This provides what look to me like the correct params, and the 'Debug' field updates as it should:

fridge: !ruby/hash:ActionController::Parameters
      debug: ''
      ingredient_ids:
      - '1'
      - '2'
    commit: Update
    controller: fridges
    action: update
    id: '2'

Could it be the @fridge variable is incorrect? It does seem to pick up the correct instance of the Fridge when I've tested it. It's currently:

@fridge = current_user.fridge #using my auth method, and unavailable to 

However, when the form is submitted for this project, no Ingredients are added to the Fridge. The has-many-through is working as it should be, and Ingredients can be added and accessed in the Rails console.

Thanks for any help!