How to combine queries with the help of the "shih tzu flag gem" in Rails?

Question

I've used to shih tzu flag gem to make a queries, then the User and Article models, as:

class User < ApplicationRecord
  belongs_to :city
  ...
  has_flags 1 => :is_admin,
            2 => :is_customer 
end

class City < ApplicationRecord
  has_many :cities
  ...
end

Then, when I make a next query to stored all city with users work to me:

 city_users = City_joins(:users).where(users: {confirmed_at: nil})

but... when I make a other query to know if user is_admin it doesn't work me,

I get this errors:

city_admin = City_joins(:users).where(users: {is_admin: true}) 

=> ActiveRecord::StatementInvalid (PG::UndefinedColumn: ERROR:  column users.is_admin does not exist)

That's because it doesn't recognize the field "is_admin", but I'd like to query method has_flags (is_admin, is_customer)

If someone knows how to make queries with has_flags or scope and could help me, I would be very grateful.