models.rb

class User < ActiveRecord::Base
 
 has_many :memberships
 has_many :boards, through: :memberships
end

class Board < ActiveRecord::Base
 has_many :memberships
end

class Membership < ActiveRecord::Base
 belongs_to :user
 belongs_to :board
 def role
   return :admin if role_mask == 1
   return :member if role_mask == 4
 end
end

#how can I get here ?
current_user.boards[0].role