has_many :grandchildren trick

class Person < ActiveRecord::Base
  belongs_to :parent, class: Person
  has_many :children, class: Person, foreign_key: :parent_id
  has_many :grandchildren, class: Person, through: :children, source: :children
end

Little bit ruby

% Notation

%q[ ] # Non-interpolated String (except for \\ \[ and \])
%Q[ ] # Interpolated String (default)
%r[ ] # Interpolated Regexp (flags can appear after the closing delimiter)
%i[ ] # Non-interpolated Array of symbols, separated by whitespace
%I[ ] # Interpolated Array of symbols, separated by whitespace
%w[ ] # Non-interpolated Array of words, separated by whitespace
%W[ ] # Interpolated Array of words, separated by whitespace
%x[ ] # Interpolated shell command

Little bit about Graphs

simple - no multiarcs and loop

non-simpl(pseudog) - multiarcs and loop

---

complete - each node conect beetwen each other directly

conected - directly or nondirectli all nodes are conected

tree - conected graph without circle

spanning tree - tree with (nodes-1 == arcs), min arcs for connect all nodes

isomorphick - graph when (arcs == nodes)

sub - part of graph

ActiveSupport::Inflector

ActiveSupport::Inflector - transforms words from singular to plural, class names to table names, modularized class names to ones without, and class names to foreign keys. The default inflections for pluralization, singularization, and uncountable words are kept in inflections.rb. 

PostgreSQL: array field

If we have array field in postgresql (like `tags`), we can use it for search as well:

- search records which contain `tag`
`ruby` = ANY(tags)
- search through array with tags
`{ruby, python, java}` && tags
- all tags should be equal `ruby`
`ruby` = ALL(tags)