1 files changed, 33 insertions, 0 deletions
diff --git a/app/services/target_nutrient_calculator.rb b/app/services/target_nutrient_calculator.rb
new file mode 100644
index 0000000..e6cd378
--- /dev/null
+++ b/app/services/target_nutrient_calculator.rb
@@ -0,0 +1,33 @@
+class TargetNutrientCalculator
+  # Derive nutrient columns from the NutrientMeasurement table
+  NUTRIENT_COLUMNS = (NutrientMeasurement.column_names - %w[id measured_on created_at updated_at])
+                      .map!(&:to_sym)
+                      .freeze
+
+  # Returns an unsaved NutrientMeasurement with target concentrations (e.g., mg/L)
+  def self.call
+    rafts = Raft.includes(:crop).where.not(crop_id: nil)
+    total = rafts.count
+    return empty_measurement if total.zero?
+
+    sums = Hash.new(0.0)
+
+    rafts.each do |raft|
+      NUTRIENT_COLUMNS.each do |col|
+        v = raft.crop.public_send(col)
+        sums[col] += v.to_f if v
+      end
+    end
+
+    targets = sums.transform_values { |s| s / total }
+    NutrientMeasurement.new({ measured_on: Date.current }.merge(targets))
+  end
+
+  private
+
+  def empty_measurement
+    NutrientMeasurement.new(
+      { measured_on: Date.current }.merge(NUTRIENT_COLUMNS.index_with { 0.0 })
+    )
+  end
+end