How to insert a condition for scope, during where?

Question

Ruby on Rails

For example, there is this code

scope :age, -> { age >= 99 }

Person.where(name: 'Alex').age

How to put a condition here?
If for example True then Person.where(name: 'Alex').age , if False then without scope Person.where(name: 'Alex')

Person.where(name: 'Alex').filter

def filter

  if expression
    age #scope
  end

end

It is clear that the code above will not work, but I need something like this.

Answer 1

I already answered almost the same question How to return all Models if id is nil in Rails?