WIP: Add virtual depth column support

- find way to setup the sql in tests - currently feels so close as this will
  basically match ancestry_depth_sql. But we have a chicken/egg issue with class and table
  may extract to a static method?
- determine if we want to un-deprecate depth_cache_column.
  I wanted to remove because column and option had different spelling, and the extra field didn't buy us too much.
  maybe hardcoding the virtual column name is good enough for now?

lib/ancestry/has_ancestry.rb

@@ -70,24 +70,29 @@ def has_ancestry options = {}
 
       # Create ancestry column accessor and set to option or default
       if options[:cache_depth]
-        # Create accessor for column name and set to option or default
-        self.cattr_accessor :depth_cache_column
-        self.depth_cache_column =
-          if options[:cache_depth] == true
-            options[:depth_cache_column]&.to_s || 'ancestry_depth'
-          else
-            options[:cache_depth].to_s
+        if options[:cache_depth] == :virtual
+          # local variable
+          depth_cache_column = 'ancestry_depth'
+        else
+          # Create accessor for column name and set to option or default
+          self.cattr_accessor :depth_cache_column
+          self.depth_cache_column =
+            if options[:cache_depth] == true
+              options[:depth_cache_column]&.to_s || 'ancestry_depth'
+            else
+              options[:cache_depth].to_s
+            end
+          if options[:depth_cache_column]
+            ActiveSupport::Deprecation.warn("has_ancestry :depth_cache_column is deprecated. Use :cache_depth instead.")
           end
-        if options[:depth_cache_column]
-          ActiveSupport::Deprecation.warn("has_ancestry :depth_cache_column is deprecated. Use :cache_depth instead.")
-        end
 
-        # Cache depth in depth cache column before save
-        before_validation :cache_depth
-        before_save :cache_depth
+          # Cache depth in depth cache column before save
+          before_validation :cache_depth
+          before_save :cache_depth
 
-        # Validate depth column
-        validates_numericality_of depth_cache_column, :greater_than_or_equal_to => 0, :only_integer => true, :allow_nil => false
+          # Validate depth column
+          validates_numericality_of depth_cache_column, :greater_than_or_equal_to => 0, :only_integer => true, :allow_nil => false
+        end
 
         scope :before_depth, lambda { |depth| where("#{depth_cache_column} < ?", depth) }
         scope :to_depth,     lambda { |depth| where("#{depth_cache_column} <= ?", depth) }

test/environment.rb

@@ -126,7 +126,17 @@ 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
+        # TODO: find a way to use Ancestry::MaterializedPath{,2}#ancestry_depth_sql
+        t.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