Find a model by a custom property

I have a model which has a custom property:

class User < ActiveRecord
  def my_custom_field
    # ....
  end
end

I need to find a user which has a certain my_custom_field. I can't do this:

User.where(my_custom_field: '123')[0]

However, I can do this:

User.all.select { |x| x == '123'}[0]

but it's not efficient performance-wise. What's a more effective way?