Set constraints on Readouts.user_id foreign key

Add Measurements table

db/migrate/20230602185352_create_units.rb

@@ -10,5 +10,6 @@ class CreateUnits < ActiveRecord::Migration[8.1]
       t.timestamps
     end
     add_index :units, [:user_id, :symbol]
+    add_index :units, [:id, :user_id]
   end
 end

db/migrate/20250104194343_create_quantities.rb

@@ -13,5 +13,6 @@ class CreateQuantities < ActiveRecord::Migration[8.1]
       t.string :pathname, null: false, limit: 511
     end
     add_index :quantities, [:user_id, :parent_id, :name]
+    add_index :quantities, [:id, :user_id]
   end
 end

db/migrate/20250121230456_create_readouts.rb

@@ -1,16 +1,63 @@
 class CreateReadouts < ActiveRecord::Migration[8.1]
   def change
-    create_table :readouts do |t|
-      t.references :user, null: false, foreign_key: {on_delete: :cascade}
-      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
+    create_table :measurements do |t|
+      t.datetime :taken_at, null: false
       #t.references :collector, foreign_key: true
       #t.references :device, foreign_key: true
+      #t.references :note, foreign_key: true
 
       t.timestamps
     end
+    add_index :measurements, :taken_at
+
+    # Defining Readouts as a super-/subclass polymorphic relations for different
+    # subclass data types (numeric, string, file) is not possible with proper
+    # referential integrity constraints. The required constraints are:
+    # * for every subclass record to have superclass record,
+    # * for every superclass record to have only one type of subclass record,
+    # * for every superclass record to have subclass record (unenforcable).
+    #   * this can be partially remedied by making superlass an abstract class in
+    #     Rails and disallow direct creation of records, but direct data
+    #     manipulation can still break referential integrity.
+    # Defining separate {Numeric,Text,File}_Readouts tables would make the
+    # unique index constraint unenforcable.
+    create_table :readouts do |t|
+      t.references :user, null: false, foreign_key: {on_delete: :cascade}
+      t.references :measurement, foreign_key: {on_delete: :cascade}
+      t.references :quantity, null: false, foreign_key: {on_delete: :cascade}
+      t.integer :category, null: false, default: 0
+      t.float :value, null: false, limit: Float::MANT_DIG
+      t.references :unit, null: false, foreign_key: {on_delete: :cascade}
+      # TODO: consider additional columns to allow wider range of value types
+      # t.text :text
+      # t.datetime :time
+      # t.references :file
+      # Possibly mutually exclusive with :unit or check constraint for:
+      #   :unit is not null <=> :value is not null
+
+      t.timestamps
+    end
+    add_index :readouts, [:measurement_id, :quantity_id, :category]
+    add_foreign_key :readouts, :quantities, column: [:quantity_id, :user_id],
+      primary_key: [:id, :user_id]
+    add_foreign_key :readouts, :units, column: [:unit_id, :user_id],
+      primary_key: [:id, :user_id]
+
+    # TODO: remove below tables after current setup verified
+    #create_table :numeric_values do |t|
+    #  t.references :readout, null: false, foreign_key: {on_delete: :cascade}
+    #  t.float :value, null: false, limit: Float::MANT_DIG
+    #  t.references :unit, null: false, foreign_key: {on_delete: :cascade}
+    #  # + generated, not stored column :value_type
+    #  # + foreign key constraint to readouts: [:readout_id, :value_id, :value_type]
+    #  # or 2 constraints: [:readout_id, :value_id], [:value_id, :value_type]
+    #  # if readouts.value_id needed, otherwise just one constraint:
+    #  # [:readout_id, :value_type]
+    #end
+
+    #create_table :string_values do |t|
+    #  t.references :readout, null: false, foreign_key: {on_delete: :cascade}
+    #  t.string :value, null: false, limit: 32
+    #end
   end
 end