How to properly add brackets to SQL queries with 'or' and 'and' clauses by using Arel?

I had the same problem. I was searching the web for some hours and finally found a method named grouping in Arel::FactoryMethods which simply adds brackets around an expression.

You should wrap your groups with a arel_table.grouping(...) call.

Example of how to use arel_table.grouping(...) as part of scope

# app/model/candy.rb
class Candy < ActiveRecord::Base
  has_many :candy_ownerships
  has_many :clients, through: :candy_ownerships, source: :owner, source_type: 'Client'
  has_many :users,   through: :candy_ownerships, source: :owner, source_type: 'User'

  # ....
  scope :for_user_or_global, ->(user) do
    # ->() is new lambda syntax, lamdba{|user| ....}

    worldwide_candies  = where(type: 'WorldwideCandies').where_values.reduce(:and)
    client_candies     = where(type: 'ClientCandies', candy_ownerships: { owner_id: user.client.id, owner_type: 'Client'}).where_values.reduce(:and)
    user_candies       = where(type: 'UserCandies',   candy_ownerships: { owner_id: user.id,        owner_type: 'User'  }).where_values.reduce(:and)

    joins(:candy_ownerships).where( worldwide_candies.or( arel_table.grouping(client_candies) ).or( arel_table.grouping(user_candies) ) )
  end

  # ....
end

call

Candy.for_user_or_global(User.last)
#=> SELECT `candies`.* FROM `candies` INNER JOIN `candy_ownerships` ON `candy_ownerships`.`candy_id` = `candies`.`id` WHERE (`candies`.`deleted_at` IS NULL) AND (((`candies`.`type` = 'WorldwideCandies' OR (`candies`.`type` = 'ClientCandies' AND `candy_ownerships`.`owner_id` = 19 AND `candy_ownerships`.`owner_type` = 'Client')) OR (`candies`.`type` = 'UserCandies' AND `candy_ownerships`.`owner_id` = 121 AND `candy_ownerships`.`owner_type` = 'User')))

thx micha for the tip