app/models/unit.rb

@@ -13,7 +13,7 @@ class Unit < ApplicationRecord
     length: {maximum: columns_hash['symbol'].limit}
   validates :description, length: {maximum: columns_hash['description'].limit}
   validates :multiplier, numericality: {equal_to: 1}, unless: :base
-  validates :multiplier, numericality: {other_than: 0}, if: :base
+  validates :multiplier, numericality: {other_than: 0, precision: true, scale: true}, if: :base
 
   scope :defaults, ->{ where(user: nil) }
   scope :defaults_diff, ->{

app/views/units/_form.html.erb

@@ -13,7 +13,8 @@
     <td>
       <% unless @unit.base.nil? %>
         <%= form.hidden_field :base_id, form: :unit_form %>
-        <%= form.number_field :multiplier, form: :unit_form, required: true, step: "any",
+        <%= form.number_field :multiplier, form: :unit_form, required: true,
+          step: BigDecimal(10).power(-@unit.class.type_for_attribute(:multiplier).scale),
           size: 10, autocomplete: "off" %>
       <% end %>
     </td>

config/initializers/core_ext.rb

@@ -1,5 +1,9 @@
 require 'core_ext/big_decimal_scientific_notation'
 
+ActiveSupport.on_load :active_record do
+  ActiveModel::Validations::NumericalityValidator.prepend CoreExt::ActiveModel::Validations::NumericalityValidatesPrecisionAndScale
+end
+
 ActiveSupport.on_load :action_dispatch_system_test_case do
   prepend CoreExt::ActionDispatch::SystemTesting::TestHelpers::ScreenshotHelperUniqueId
 end

config/locales/en.yml

@@ -1,4 +1,8 @@
 en:
+  errors:
+    messages:
+      precision_exceeded: must not exceed %{value} significant digits
+      scale_exceeded: must not exceed %{value} decimal digits
   activerecord:
     attributes:
       unit:

lib/core_ext/active_model/validations/numericality_validates_precision_and_scale.rb

@@ -0,0 +1,16 @@
+module CoreExt::ActiveModel::Validations::NumericalityValidatesPrecisionAndScale
+  def validate_each(record, attr_name, value, ...)
+    super(record, attr_name, value, ...)
+
+    if options[:precision] || options[:scale]
+      attr_type = record.class.type_for_attribute(attr_name)
+      value = BigDecimal(value) unless value.is_a? BigDecimal
+      if options[:precision] && (value.precision > attr_type.precision)
+        record.errors.add(attr_name, :precision_exceeded, **filtered_options(attr_type.precision))
+      end
+      if options[:scale] && (value.scale > attr_type.scale)
+        record.errors.add(attr_name, :scale_exceeded, **filtered_options(attr_type.scale))
+      end
+    end
+  end
+end