RailsのArelを調査してみた ref: https://qiita.com/valley/items/9ac200acbccc46b2ea44

binary.rb

# Arel::Noees::Orの親クラス
def initialize left, right
  super()
  @left  = left
  @right = right
end

file0.rb

users = Arel::Table.new(:users)
query = users.project("id").where(users[:age].gt(20).and(users[:age].lt(30))).order("id")
query.to_sql
# "SELECT id FROM \"users\" WHERE \"users\".\"age\" > 20 AND \"users\".\"age\" < 30 ORDER BY id"

file4.rb

new_node = left_node.create_node(right_node)
new_node.left == left_node # true
new_node.right == right_node # true

file5.rb

users = Arel::Table.new(:usrs)
users.where(
  users[:age].eq(20).or(users[:age].eq(18)).and(users[:age].gt(15))
)

file6.sql

users.age = 20 OR users.age =18 AND users.age > 15

file7.rb

users.from.to_sql # "SELECT FROM \"users\""
users[:age].eq(20).or(users[:age].eq(18)).and(users[:age].gt(15)).to_sql
# "(\"users\".\"age\" = 20 OR \"users\".\"age\" = 18) AND \"users\".\"age\" > 15"

file9.rb

eql_node = users[:age].eq(20)
eql_node.class # Arel::Nodes::Equality
eql_node.left
# <struct Arel::Attributes::Attribute relation=#<Arel::Table:0x007fd378afb1c8 @name="users", @type_caster=nil, @table_alias=nil>, name=:age>

eql_node.right
# <Arel::Nodes::Casted:0x007fd37a401118 @val=20, @attribute=#<struct Arel::Attributes::Attribute relation=#<Arel::Table:0x007fd378afb1c8 @name="users", @type_caster=nil, @table_alias=nil>, name=:age>>

node.rb

def or right
  Nodes::Grouping.new Nodes::Or.new(self, right)
end

select_manager.rb

def project *projections
  # FIXME: converting these to SQLLiterals is probably not good, but
  # rails tests require it.
  @ctx.projections.concat projections.map { |x|
    STRING_OR_SYMBOL_CLASS.include?(x.class) ? Nodes::SqlLiteral.new(x.to_s) : x
  }
  self #　自分を返している = チェーン可能
end

table.rb

def from
  SelectManager.new(self)
end

def where condition
  from.where condition
end

def project *things
  from.project(*things)
end

tree_manager.rb

# Arel::SelectManagerの親クラス
def where expr
  if Arel::TreeManager === expr
    expr = expr.ast
  end
  @ctx.wheres << expr
  self #　自分を返している = チェーン可能
end

ちょっと成長した木

users[:age].eq(20).or(users[:age].eq(18))

もっと成長した木

users[:age].eq(20).or(users[:age].eq(18)).and(users[:age].gt(15))

小さな木

users[:age].eq(20)