Adding Postgres Cycle detection

Author: wollistik

CHANGELOG.md

@@ -1,5 +1,6 @@
 ### NEXT
-- Added :dependent option for setting explicit 
+- Added :dependent option for setting explicit deletion behaviour (issue #31)
+- Added automatic cycle detection when supported (currently only PostgresSQL 14+) (issue #22)
 
 ### Version 3.4.0
 - Rails 7.1 compatibility

README.md

@@ -224,6 +224,15 @@ Instance Methods make no difference of the class from which they are called:
 sub_node_instance.descendants # => returns Node and SubNode instances
 ```
 
+## A note on endless recursion / cycle detection
+
+### Inserting
+As of now it is up to the user code to guarantee there will be no cycles created in the parent/child entries. If not, your DB might run into an endless recursion. Inserting/updating records that will cause a cycle is not prevented by some validation checks, so you have to do this by your own. This might change in a future version.
+
+### Querying
+If you want to make sure to not run into an endless recursion when querying, then there are following options:
+1. Add a maximum depth to the query options. If an cycle is present in your data, the recursion will stop when reaching the max depth and stop further traversing.
+2. When you are on recent version of PostgreSQL (14+) you are lucky. Postgres added the CYCLE detection feature to detect cycles and prevent endless recursion. Our query builder will add this feature if your DB does support this.  
 
 ## Contributing
 

lib/acts_as_recursive_tree/builders/ancestors.rb

@@ -5,7 +5,7 @@ module Builders
     class Ancestors < RelationBuilder
       self.traversal_strategy = ActsAsRecursiveTree::Builders::Strategies::Ancestor
 
-      def get_query_options(_)
+      def get_query_options(&block)
         opts = super
         opts.ensure_ordering!
         opts

lib/acts_as_recursive_tree/builders/leaves.rb

@@ -16,7 +16,7 @@ def create_select_manger(column = nil)
         select_manager
       end
 
-      def get_query_options(_)
+      def get_query_options(&_block)
         # do not allow any custom options
         ActsAsRecursiveTree::Options::QueryOptions.new
       end

lib/acts_as_recursive_tree/builders/relation_builder.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'securerandom'
+
 module ActsAsRecursiveTree
   module Builders
     #
@@ -12,40 +14,42 @@ def self.build(klass, ids, exclude_ids: false, &block)
 
       class_attribute :traversal_strategy, instance_writer: false
 
-      attr_reader :klass, :ids, :recursive_temp_table, :travers_loc_table, :without_ids
-
-      mattr_reader(:random) { Random.new }
+      attr_reader :klass, :ids, :without_ids
 
       # Delegators for easier accessing config and query options
-      delegate :primary_key, :depth_column, :parent_key, :parent_type_column, to: :@config
+      delegate :primary_key, :depth_column, :parent_key, :parent_type_column, to: :config
       delegate :depth_present?, :depth, :condition, :ensure_ordering, to: :@query_opts
 
       def initialize(klass, ids, exclude_ids: false, &block)
         @klass       = klass
-        @config      = klass._recursive_tree_config
-        @ids         = ActsAsRecursiveTree::Options::Values.create(ids, @config)
+        @ids         = ActsAsRecursiveTree::Options::Values.create(ids, klass._recursive_tree_config)
         @without_ids = exclude_ids
 
-        @query_opts = get_query_options(block)
+        @query_opts = get_query_options(&block)
+
+        # random seed for the temp tables
+        @rand_int = SecureRandom.rand(1_000_000)
+      end
+
+      def recursive_temp_table
+        @recursive_temp_table ||= Arel::Table.new("recursive_#{klass.table_name}_#{@rand_int}_temp")
+      end
+
+      def travers_loc_table
+        @travers_loc_table ||= Arel::Table.new("traverse_#{@rand_int}_loc")
+      end
 
-        rand_int              = random.rand(1_000_000)
-        @recursive_temp_table = Arel::Table.new("recursive_#{klass.table_name}_#{rand_int}_temp")
-        @travers_loc_table    = Arel::Table.new("traverse_#{rand_int}_loc")
+      def config
+        klass._recursive_tree_config
       end
 
       #
       # Constructs a new QueryOptions and yield it to the proc if one is present.
       # Subclasses may override this method to provide sane defaults.
       #
-      # @param proc [Proc] a proc or nil
-      #
       # @return [ActsAsRecursiveTree::Options::QueryOptions] the new QueryOptions instance
-      def get_query_options(proc)
-        opts = ActsAsRecursiveTree::Options::QueryOptions.new
-
-        proc&.call(opts)
-
-        opts
+      def get_query_options(&block)
+        ActsAsRecursiveTree::Options::QueryOptions.from(&block)
       end
 
       def base_table
@@ -71,11 +75,7 @@ def apply_depth(select_manager)
       end
 
       def create_select_manger(column = nil)
-        projections = if column
-                        travers_loc_table[column]
-                      else
-                        Arel.star
-                      end
+        projections = column ? travers_loc_table[column] : Arel.star
 
         select_mgr = travers_loc_table.project(projections).with(:recursive, build_cte_table)
 
@@ -85,10 +85,24 @@ def create_select_manger(column = nil)
       def build_cte_table
         Arel::Nodes::As.new(
           travers_loc_table,
-          build_base_select.union(build_union_select)
+          add_pg_cycle_detection(
+            build_base_select.union(build_union_select)
+          )
+        )
+      end
+
+      def add_pg_cycle_detection(union_query)
+        return union_query unless config.cycle_detection?
+
+        Arel::Nodes::InfixOperation.new(
+          '',
+          union_query,
+          Arel.sql("CYCLE #{primary_key} SET is_cycle USING path")
         )
       end
 
+      # Builds SQL:
+      # SELECT id, parent_id, 0 AS depth FROM base_table WHERE id = 123
       def build_base_select
         id_node = base_table[primary_key]
 

lib/acts_as_recursive_tree/config.rb

@@ -21,5 +21,15 @@ def initialize(model_class:, parent_key:, parent_type_column:, depth_column: :re
     def primary_key
       @primary_key ||= @model_class.primary_key.to_sym
     end
+
+    #
+    # Checks if SQL cycle detection can be used. This is currently supported only on PostgreSQL 14+.
+    # @return [TrueClass|FalseClass]
+    def cycle_detection?
+      return @cycle_detection if defined?(@cycle_detection)
+
+      @cycle_detection = @model_class.connection.adapter_name == 'PostgreSQL' &&
+                         @model_class.connection.database_version >= 140_000
+    end
   end
 end

lib/acts_as_recursive_tree/options/query_options.rb

@@ -5,6 +5,12 @@ module Options
     class QueryOptions
       STRATEGIES = %i[subselect join].freeze
 
+      def self.from
+        options = new
+        yield(options) if block_given?
+        options
+      end
+
       attr_accessor :condition
       attr_reader :ensure_ordering, :query_strategy