Drop Readout.value decimal type in favor of float

DESIGN.md

@@ -0,0 +1,34 @@
+DESIGN
+======
+
+Below is a list of design decisions. The justification is to be consulted
+whenever a change is considered, to avoid regressions.
+
+### Data type for DB storage of numeric values (`decimal` vs `float`)
+
+* among database engines supported (by Rails), SQLite offers storage of
+  `decimal` data type with the lowest precision, equal to the precision of
+  `REAL` type (double precision float value, IEEE 754), but in a floating point
+  format,
+    * decimal types in other database engines offer greater precision, but store
+      data in a fixed point format,
+* biology-related values differ by several orders of magnitude; storing them in
+  fixed point format would only make sense if required precision would be
+  greater than that offered by floating point format,
+    * even then, fixed point would mean either bigger memory requirements or
+      worse precision for numbers close to scale limit,
+        * for a fixed point format to use the same 8 bytes of storage as IEEE
+          754, precision would need to be limited to 18 digits (4 bytes/9 digits)
+          and scale approximately half of that - 9,
+    * double precision floating point guarantees 15 digits of precision, which
+      is more than enough for all expected use cases,
+        * single precision floating point only guarntees 6 digits of precision,
+          which is estimated to be too low for some use cases (e.g. storing
+          latitude/longitude with a resolution grater than 100m)
+* double precision floating point (IEEE 754) is a standard that ensures
+  compatibility with all database engines,
+    * the same data format is used internally by Ruby as a `Float`; it
+      guarantees no conversions between storage and computation,
+    * as a standard with hardware implementations ensures both: computing
+      efficiency and hardware/3rd party library compatibility as opposed to Ruby
+      custom `BigDecimal` type

app/helpers/application_helper.rb

@@ -102,13 +102,17 @@ module ApplicationHelper
 
     def number_field(method, options = {})
       attr_type = object.type_for_attribute(method)
-      if attr_type.type == :decimal
+      case attr_type.type
+      when :decimal
         options[:value] = object.public_send(method)&.to_scientific
         options[:step] ||= BigDecimal(10).power(-attr_type.scale)
         options[:max] ||= BigDecimal(10).power(attr_type.precision - attr_type.scale) -
           options[:step]
         options[:min] = options[:min] == :step ? options[:step] : options[:min]
         options[:min] ||= -options[:max]
+        options[:size] ||= attr_type.precision / 2
+      when :float
+        options[:size] ||= 6
       end
       super
     end

app/views/readouts/_form.html.erb

@@ -11,9 +11,7 @@
       <%= readout.quantity.relative_pathname(@superquantity) %>
     </td>
     <td>
-      <%= form.number_field :value, required: true,
-        size: readout.type_for_attribute(:value).precision / 2,
-        autofocus: readout_counter == 0 %>
+      <%= form.number_field :value, required: true, autofocus: readout_counter == 0 %>
     </td>
     <td>
       <%= form.hidden_field :quantity_id %>

db/migrate/20250121230456_create_readouts.rb

@@ -1,10 +1,14 @@
 class CreateReadouts < ActiveRecord::Migration[7.2]
   def change
     create_table :readouts do |t|
-      t.references :user, null: false, foreign_key: true
+      # Reference :user through :quantity (:measurement may be NULL).
+      t.references :measurement, foreign_key: true
       t.references :quantity, null: false, foreign_key: true
+      # :category + :value + :unit as a separate table? (NumericValue, TextValue)
+      t.integer :category, null: false, default: 0
+      t.float :value, null: false, limit: Float::MANT_DIG
       t.references :unit, foreign_key: true
-      t.decimal :value, null: false, precision: 30, scale: 15
+      # Move to Measurement?
       #t.references :collector, foreign_key: true
       #t.references :device, foreign_key: true