query.rb

class User < ApplicationRecord
  has_many :wallets
end

class Currency < ApplicationRecord
  has_many :wallets
end

class Wallet < ApplicationRecord
  belongs_to :currency
  belongs_to :user
end

Currency.create(currency_symbol: "usd")
Currency.create(currency_symbol: "eur")

10.times do 
  user = User.create;
  user.wallets.create(currency_symbol: "usd")
  user.wallets.create(currency_symbol: "eur")
end

# Let's assume not all wallets were created and there's one eur wallet missing, what is the query to find it? 
# Something like this but this is not working.

#
User.joins(wallets: :currency).where(currency_symbol: "eur", wallets: { currency: { id: nil } })