Updated comments, added before_destroy where applicable

app/models/ingredient.rb

@@ -6,6 +6,12 @@ class Ingredient < ActiveRecord::Base
     meat: 1
   }
 
+  # Has to go before any 'dependent:' association
+  before_destroy do
+    # FIXME: disallow destruction if any object depends on this quantity
+    nil
+  end
+
   belongs_to :project, required: true
   belongs_to :ref_unit, class_name: 'Unit', required: true
   belongs_to :source, required: false

app/models/measurement.rb

@@ -7,7 +7,8 @@ class Measurement < ActiveRecord::Base
   accepts_nested_attributes_for :readouts, allow_destroy: true, reject_if: proc { |attrs|
     attrs['quantity_id'].blank? && attrs['value'].blank?
   }
-  # Readout (quantity_id, unit_id) pair uniqueness check for nested attributes
+  # Readout quantity_id + unit_id uniqueness validation. Cannot be effectively
+  # checked on Readout model level.
   validate do
     quantities = self.readouts.map { |r| [r.quantity_id, r.unit_id] }
     if quantities.length != quantities.uniq.length

app/models/quantity.rb

@@ -7,6 +7,12 @@ class Quantity < ActiveRecord::Base
     exercise: 2
   }
 
+  # Has to go before any 'dependent:' association
+  before_destroy do
+    # FIXME: disallow destruction if any object depends on this quantity
+    nil
+  end
+
   acts_as_nested_set dependent: :destroy, scope: :project
   belongs_to :project, required: false
 

app/models/source.rb

@@ -2,4 +2,10 @@ class Source < ActiveRecord::Base
   belongs_to :project, required: false
 
   validates :name, presence: true, uniqueness: {scope: :project_id}
+
+  # Has to go before any 'dependent:' association
+  before_destroy do
+    # FIXME: disallow destruction if any object depends on this quantity
+    nil
+  end
 end

app/models/unit.rb

@@ -2,4 +2,10 @@ class Unit < ActiveRecord::Base
   belongs_to :project, required: false
 
   validates :shortname, presence: true, uniqueness: {scope: :project_id}
+
+  # Has to go before any 'dependent:' association
+  before_destroy do
+    # FIXME: disallow destruction if any object depends on this quantity
+    nil
+  end
 end