Arel composition (without ActiveRecord)

arel_composition_without_activerecord.rb

# rebuttal of http://www.try-alf.org/blog/2013-10-21-relations-as-first-class-citizen

require 'arel'
require 'sqlite3'
require 'pry'

def suppliers
  @suppliers ||= Arel::Table.new(:suppliers)
end

def cities
  @cities ||= Arel::Table.new(:cities)
end

def suppliers_in(city)
  suppliers.project(suppliers[:sid], suppliers[:name], suppliers[:city])
           .where(suppliers[:city].eq city)
end

def with_country(operand)
  operand.join(cities).project(cities[:country])
end

# with_country(suppliers_in('Paris')).to_sql
# => "SELECT `suppliers`.`sid`, `suppliers`.`name`, `suppliers`.`city`, `cities`.`country` FROM `suppliers` INNER JOIN `cities`  WHERE `suppliers`.`city` = \"Paris\""


# minimal hack of a sqlite adapter

Column = Struct.new(:name, :default, :cast_type, :sql_type, :null)

module SchemaCache
  def schema_cache
    self
  end

  def table_exists?(name)
    tables.include?(name)
  end

  def tables
    @tables ||= execute(<<-SQL).flatten
      SELECT name FROM sqlite_master
      WHERE type = 'table' AND NOT name = 'sqlite_sequence'
    SQL
  end

  def columns(table_name)
    schema = execute(<<-SQL)
      PRAGMA table_info(#{table_name})
    SQL

    @columns ||= schema.map do |_, name, type, not_null, default, _|
      Column.new(name, default == 'null' ? nil : default, String, type, not_null == 0)
    end
  end

  def columns_hash
    @columns_hash ||= Hash.new do |h, table_name|
      h[table_name] = Hash[columns(table_name).map { |col|
        [col.name, col]
      }]
    end
  end
end

module Quotes
  def quote_table_name(name)
    "`#{name}`"
  end

  def quote_column_name(name)
    "`#{name}`"
  end

  def quote(value, column)
    "\"#{value}\""
  end
end

module Visitable
  def visitor
    @visitor ||= Arel::Visitors::SQLite.new(self)
  end
end

$connection = SQLite3::Database.new(':memory:')
$connection.extend SchemaCache
$connection.extend Quotes
$connection.extend Visitable

$connection.execute(<<-SQL)
  CREATE TABLE suppliers
  (sid int, name varchar(255), status int, city varchar(255))
SQL

$connection.execute(<<-SQL)
  CREATE TABLE cities
  (city varchar(255), country varchar(255))
SQL

Arel::Table.engine = Class.new do
  def connection
    $connection
  end
end.new

binding.pry