diff --git a/app/extensions/tree_node_extension.rb b/app/extensions/tree_node_extension.rb
new file mode 100644
index 0000000..fe4f720
--- /dev/null
+++ b/app/extensions/tree_node_extension.rb
@@ -0,0 +1,18 @@
+module TreeNodeExtension
+  def distance_to_common_ancestor(another_node)
+    return 0 if self == another_node
+
+    my_path = path_as_array
+    another_path = another_node.path_as_array
+
+    common_elements = my_path.zip(another_path)
+                             .take_while { |(p1, p2)| p1 == p2 }
+                             .count
+
+    return nil if common_elements.zero?
+
+    [my_path.count, another_path.count].max - common_elements
+  end
+end
+
+Tree::TreeNode.include(TreeNodeExtension)
diff --git a/config/application.rb b/config/application.rb
index e21736e..39a83b3 100644
--- a/config/application.rb
+++ b/config/application.rb
@@ -28,6 +28,8 @@ module WeddingPlanner
     # Common ones are `templates`, `generators`, or `middleware`, for example.
     config.autoload_lib(ignore: %w[assets tasks])
 
+    config.eager_load_paths << root.join('extras')
+
     # Configuration for the application, engines, and railties goes here.
     #
     # These settings can be overridden in specific environments using the files
diff --git a/config/environments/development.rb b/config/environments/development.rb
index 2e7fb48..794f390 100644
--- a/config/environments/development.rb
+++ b/config/environments/development.rb
@@ -9,7 +9,7 @@ Rails.application.configure do
   config.enable_reloading = true
 
   # Do not eager load code on boot.
-  config.eager_load = false
+  config.eager_load = true
 
   # Show full error reports.
   config.consider_all_requests_local = true
diff --git a/config/environments/test.rb b/config/environments/test.rb
index adbb4a6..3ada93b 100644
--- a/config/environments/test.rb
+++ b/config/environments/test.rb
@@ -15,7 +15,7 @@ Rails.application.configure do
   # this is usually not necessary, and can slow down your test suite. However, it's
   # recommended that you enable it in continuous integration systems to ensure eager
   # loading is working properly before deploying your code.
-  config.eager_load = ENV["CI"].present?
+  config.eager_load = true
 
   # Configure public file server for tests with Cache-Control for performance.
   config.public_file_server.enabled = true
diff --git a/spec/extensions/tree_spec.rb b/spec/extensions/tree_spec.rb
new file mode 100644
index 0000000..cbefba4
--- /dev/null
+++ b/spec/extensions/tree_spec.rb
@@ -0,0 +1,71 @@
+require 'rails_helper'
+
+module Tree
+  RSpec.describe TreeNode do
+    describe '#distance_to_common_ancestor' do
+      def assert_distance(node_1, node_2, distance)
+        aggregate_failures do
+          expect(node_1.distance_to_common_ancestor(node_2)).to eq(distance)
+          expect(node_2.distance_to_common_ancestor(node_1)).to eq(distance)
+        end
+      end
+      context 'when the two nodes are the same' do
+        it 'returns 0 when comparing the root itself' do
+          root = Tree::TreeNode.new('root')
+          assert_distance(root, root, 0)
+        end
+
+        it 'returns 0 when comparing a child to itself' do
+          root = Tree::TreeNode.new('root')
+          child = root << Tree::TreeNode.new('child')
+          assert_distance(child, child, 0)
+        end
+      end
+
+      context 'when the two nodes are siblings' do
+        it 'returns 1 when comparing siblings' do
+          root = Tree::TreeNode.new('root')
+          child1 = root << Tree::TreeNode.new('child1')
+          child2 = root << Tree::TreeNode.new('child2')
+          assert_distance(child1, child2, 1)
+        end
+      end
+
+      context 'when one node is parent of the other' do
+        it 'returns 1 when comparing parent to child' do
+          root = Tree::TreeNode.new('root')
+          child = root << Tree::TreeNode.new('child')
+          assert_distance(root, child, 1)
+        end
+      end
+
+      context 'when one node is grandparent of the other' do
+        it 'returns 2 when comparing grandparent to grandchild' do
+          root = Tree::TreeNode.new('root')
+          child = root << Tree::TreeNode.new('child')
+          grandchild = child << Tree::TreeNode.new('grandchild')
+          assert_distance(root, grandchild, 2)
+        end
+      end
+
+      context 'when the two nodes are cousins' do
+        it 'returns 2 when comparing cousins' do
+          root = Tree::TreeNode.new('root')
+          child1 = root << Tree::TreeNode.new('child1')
+          child2 = root << Tree::TreeNode.new('child2')
+          grandchild1 = child1 << Tree::TreeNode.new('grandchild1')
+          grandchild2 = child2 << Tree::TreeNode.new('grandchild2')
+          assert_distance(grandchild1, grandchild2, 2)
+        end
+      end
+
+      context 'when the two nodes are not related' do
+        it 'returns nil' do
+          root = Tree::TreeNode.new('root')
+          another_root = Tree::TreeNode.new('another_root')
+          assert_distance(root, another_root, nil)
+        end
+      end
+    end
+  end
+end