gistfile1.rb

class Company < ActiveRecord::Base
  has_many :company_accounts
  has_many :users, through: :company_accounts
end

class User < ActiveRecord::Base
  has_many :company_accounts
  has_many :companies, through: :company_accounts
end

#has a user_id, company_id and a manager column. If manager is set the user is able to edit the company.
class CompanyAccount < ActiveRecord::Base
  belongs_to :user
  belongs_to :company
end

#Enter CanCan, in the ability class I'm now doing:
  can :read, Company do |company|
    company.users.include?(user) && user.company_accounts.where(company_id: company.id).first.manager
  end
  # I would like to be able to do something like this:
  can :read, Company, company_account: {user_id: user.id}
  #this returns 
  "SELECT `companies`.* FROM `companies`  WHERE `company_account`.`user_id` = 1"
  

  #Update:
  #I'm able to do something like this:
  can :manage, Company, Company.joins(:company_accounts => :user).where(:company_accounts => { :user_id => user.id, :manager => true })
  # This doesn't raise a cancan exception though.