Fix performance issue when getting descendants.

[Gee-Bee]

When building tree, database is hitted for each node, looking up its parent node. The culprit is here:

acts_as_sane_tree/lib/acts_as_sane_tree/singleton_methods.rb

Lines 150 to 151 in d7aa2db

	if(cache[item.parent])
	cache[item.parent][item] = values

.
My branch fixes this issue.

This is example output before:

pry(main)> PWD.nodes_and_descendants(to_depth: 2)                                                                                                                [88/1827]
  PWD Load (3.3ms)  SELECT "pwds".* FROM (WITH RECURSIVE crumbs AS (
          SELECT pwds.*, 0 AS depth FROM pwds WHERE parent_id IS NULL
          UNION ALL
          SELECT alias1.*, crumbs.depth + 1 FROM crumbs JOIN pwds alias1 on alias1.parent_id = crumbs.id
          WHERE crumbs.depth + 1 < 2
        ) SELECT * FROM crumbs) as pwds WHERE (pwds.depth >= 0) AND (pwds.depth + 1 < 4)
  PWD Load (0.4ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.4ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.4ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.6ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.2ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.2ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.4ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.9ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.4ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.5ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.3ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.2ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]
  PWD Load (0.2ms)  SELECT  "pwds".* FROM "pwds" WHERE "pwds"."id" = $1 LIMIT 1  [["id", 3]]

and after:

pry(main)> PWD.nodes_and_descendants(to_depth: 2)
  PWD Load (1.8ms)  SELECT "pwds".* FROM (WITH RECURSIVE crumbs AS (
          SELECT pwds.*, 0 AS depth FROM pwds WHERE parent_id IS NULL
          UNION ALL
          SELECT alias1.*, crumbs.depth + 1 FROM crumbs JOIN pwds alias1 on alias1.parent_id = crumbs.id
          WHERE crumbs.depth + 1 < 2
        ) SELECT * FROM crumbs) as pwds WHERE (pwds.depth >= 0) AND (pwds.depth + 1 < 4)