Measurement form based on select-styled <details>

app/controllers/readouts_controller.rb

@@ -0,0 +1,39 @@
+class ReadoutsController < ApplicationController
+  before_action :find_quantities, only: [:new]
+  before_action :find_quantity, only: [:discard]
+  before_action :find_prev_quantities, only: [:new, :discard]
+
+  def new
+    @quantities -= @prev_quantities
+    # TODO: raise ParameterInvalid if new_quantities.empty?
+    @readouts = current_user.readouts.build(@quantities.map { |q| {quantity: q} })
+
+    @user_units = current_user.units.ordered
+
+    quantities = @prev_quantities + @quantities
+    @superquantity = current_user.quantities
+      .common_ancestors(quantities.map(&:parent_id)).first
+  end
+
+  def discard
+    @prev_quantities -= [@quantity]
+
+    @superquantity = current_user.quantities
+      .common_ancestors(@prev_quantities.map(&:parent_id)).first
+  end
+
+  private
+
+  def find_quantities
+    @quantities = current_user.quantities.find(params[:quantity])
+  end
+
+  def find_quantity
+    @quantity = current_user.quantities.find_by!(id: params[:id])
+  end
+
+  def find_prev_quantities
+    prev_quantity_ids = params[:readouts]&.map { |r| r[:quantity_id] } || []
+    @prev_quantities = current_user.quantities.find(prev_quantity_ids)
+  end
+end