diff --git a/app/models/formula.rb b/app/models/formula.rb
index 659cecc..eee0891 100644
--- a/app/models/formula.rb
+++ b/app/models/formula.rb
@@ -20,7 +20,7 @@ class Formula < ActiveRecord::Base
   def calculate(inputs)
     raise(InvalidInputs, 'No inputs') if inputs.empty?
 
-    deps = inputs.map { |q, v| [q.name, v.transpose.first] }.to_h
+    deps = inputs.map { |q, v| [q.name, QuantityInput.new(q, v.transpose.first)] }.to_h
     length = deps.values.first.length
 
     raise(InvalidFormula, 'Invalid formula') unless self.valid?
@@ -47,6 +47,10 @@ class Formula < ActiveRecord::Base
       super(*args)
       @quantity = q
     end
+
+    def lastBefore(timepoints)
+      self.map{ BigDecimal(2000) }
+    end
   end
 
   def parse
@@ -77,7 +81,7 @@ class Formula < ActiveRecord::Base
     errors
   end
 
-  def get_binding(quantities, args, length)
+  def get_binding(quantities, parts, length)
     binding
   end
 end
diff --git a/lib/body_tracking/formula_builder.rb b/lib/body_tracking/formula_builder.rb
index fe4a42b..fe38a00 100644
--- a/lib/body_tracking/formula_builder.rb
+++ b/lib/body_tracking/formula_builder.rb
@@ -177,7 +177,14 @@ module BodyTracking
         [
           :bt_expression,
           [left, right].map do |side|
-            side[0] == :bt_quantity ? "quantities['#{side[1]}'][_index]" : "#{side[1]}"
+            case side[0]
+            when :bt_quantity
+              "quantities['#{side[1]}'][_index]"
+            when :bt_quantity_method_call
+              "#{side[1]}[_index]"
+            else
+              "#{side[1]}"
+            end
           end.join(op.to_s)
         ]
       end
diff --git a/test/unit/formula_test.rb b/test/unit/formula_test.rb
index 2b588f0..ee34079 100644
--- a/test/unit/formula_test.rb
+++ b/test/unit/formula_test.rb
@@ -65,13 +65,20 @@ class FormulaTest < ActiveSupport::TestCase
 
       # Model method calls
       '100*Energy/RM.lastBefore(Meal.eaten_at||Meal.created_at)', Set['Energy', 'RM', 'Meal'],
-      # TODO: fill parts
-      []
+      [
+        {type: :indexed, content: "quantities['Meal'][_index].eaten_at||" \
+         "quantities['Meal'][_index].created_at"},
+        {type: :unindexed, content: "quantities['RM'].lastBefore(parts[0])"},
+        {type: :indexed, content: "100*quantities['Energy'][_index]/parts[1][_index]"}
+      ]
     ]
 
     d_methods = ['nil?', 'abs']
+    q_methods = Hash.new(['all', 'lastBefore'])
+    q_methods['Meal'] = ['created_at', 'eaten_at']
+
     vector.each_slice(3) do |formula, identifiers, parts|
-      parser = FormulaBuilder.new(formula, d_methods: d_methods)
+      parser = FormulaBuilder.new(formula, d_methods: d_methods, q_methods: q_methods)
       i, p = parser.parse
       assert_empty parser.errors
       assert_equal identifiers, i