Add support for virtual depth column

still need to consider whether to keep depth_cache_column as deprecated

lib/ancestry/has_ancestry.rb

@@ -64,7 +64,11 @@ def has_ancestry options = {}
       end
 
       # Create ancestry column accessor and set to option or default
-      if options[:cache_depth]
+
+      if options[:cache_depth] == :virtual
+        # NOTE: not setting self.depth_cache_column so the code does not try to update the column
+        depth_cache_sql = options[:depth_cache_column]&.to_s || 'ancestry_depth'
+      elsif options[:cache_depth]
         # Create accessor for column name and set to option or default
         self.cattr_accessor :depth_cache_column
         self.depth_cache_column =

test/concerns/depth_virtual_test.rb

@@ -0,0 +1,104 @@
+require_relative '../environment'
+
+# These are only valid for postgres 
+class DepthVirtualTest < ActiveSupport::TestCase
+  def test_depth_caching
+    return unless test_virtual_column?
+
+    AncestryTestDatabase.with_model :depth => 3, :width => 3, :cache_depth => :virtual do |_model, roots|
+      roots.each do |lvl0_node, lvl0_children|
+        assert_equal 0, lvl0_node.depth
+        lvl0_children.each do |lvl1_node, lvl1_children|
+          assert_equal 1, lvl1_node.depth
+          lvl1_children.each do |lvl2_node, _lvl2_children|
+            assert_equal 2, lvl2_node.depth
+          end
+        end
+      end
+    end
+  end
+
+  def test_depth_caching_after_subtree_movement
+    return unless test_virtual_column?
+
+    AncestryTestDatabase.with_model :depth => 6, :width => 1, :cache_depth => :virtual do |model, _roots|
+      node = model.at_depth(3).first
+      node.update(:parent => model.roots.first)
+      assert_equal(1, node.depth)
+      node.children.each do |child|
+        assert_equal(2, child.depth)
+        child.children.each do |gchild|
+          assert_equal(3, gchild.depth)
+        end
+      end
+    end
+  end
+
+  def test_depth_scopes
+    return unless test_virtual_column?
+
+    AncestryTestDatabase.with_model :depth => 4, :width => 2, :cache_depth => true do |model, _roots|
+      model.before_depth(2).all? { |node| assert node.depth < 2 }
+      model.to_depth(2).all?     { |node| assert node.depth <= 2 }
+      model.at_depth(2).all?     { |node| assert node.depth == 2 }
+      model.from_depth(2).all?   { |node| assert node.depth >= 2 }
+      model.after_depth(2).all?  { |node| assert node.depth > 2 }
+    end
+  end
+
+  def test_depth_scopes_without_depth_cache
+    return unless test_virtual_column?
+
+    AncestryTestDatabase.with_model :depth => 4, :width => 2 do |model, _roots|
+      model.before_depth(2).all? { |node| assert node.depth < 2 }
+      model.to_depth(2).all?     { |node| assert node.depth <= 2 }
+      model.at_depth(2).all?     { |node| assert node.depth == 2 }
+      model.from_depth(2).all?   { |node| assert node.depth >= 2 }
+      model.after_depth(2).all?  { |node| assert node.depth > 2 }
+    end
+  end
+
+  def test_exception_when_rebuilding_depth_cache_for_model_without_depth_caching
+    return unless test_virtual_column?
+
+    AncestryTestDatabase.with_model do |model|
+      assert_raise Ancestry::AncestryException do
+        model.rebuild_depth_cache!
+      end
+    end
+  end
+
+  def test_exception_on_unknown_depth_column
+    return unless test_virtual_column?
+
+    AncestryTestDatabase.with_model :cache_depth => true do |model|
+      assert_raise Ancestry::AncestryException do
+        model.create!.subtree(:this_is_not_a_valid_depth_option => 42)
+      end
+    end
+  end
+
+  # we are already testing generate and parse against static values
+  # this assumes those are methods are tested and working
+  def test_ancestry_depth_change
+    return unless test_virtual_column?
+
+    AncestryTestDatabase.with_model do |model|
+      {
+        [[], [1]]        => +1,
+        [[1], []]        => -1,
+        [[1], [2]]       =>  0,
+        [[1], [1, 2, 3]] => +2,
+        [[1, 2, 3], [1]] => -2
+      }.each do |(before, after), diff|
+        a_before = model.generate_ancestry(before)
+        a_after = model.generate_ancestry(after)
+        assert_equal(diff, model.ancestry_depth_change(a_before, a_after))
+      end
+    end
+  end
+
+  def test_virtual_column?
+    AncestryTestDatabase.postgres? && ActiveRecord.version.to_s >= "7.0"
+  end
+end

test/environment.rb

@@ -126,7 +126,20 @@ def self.with_model options = {}
 
     ActiveRecord::Base.connection.create_table 'test_nodes', **table_options do |table|
       table.send(column_type, options[:ancestry_column], **column_options(force_allow_nil: skip_ancestry))
-      table.integer options[:cache_depth] == true ? :ancestry_depth : options[:cache_depth] if options[:cache_depth]
+      case options[:cache_depth]
+      when true
+        table.integer :ancestry_depth
+      when :virtual
+        # sorry, this duplicates has_ancestry a little
+        path_module = Ancestry::HasAncestry.ancestry_format_module(options[:ancestry_format])
+        ancestry_depth_sql = path_module.construct_depth_sql("test_nodes", options[:ancestry_column], '/')
+
+        table.virtual :ancestry_depth, type: :integer, as: ancestry_depth_sql, stored: true
+      when nil, false
+        # no column
+      else
+        table.integer options[:cache_depth]
+      end
       if options[:counter_cache]
         counter_cache_column = options[:counter_cache] == true ? :children_count : options[:counter_cache]
         table.integer counter_cache_column, default: 0, null: false