Well, you might try something like executing each named scope that you have params for and find the intersection of the IDs of all people returned from each named_scope. That might look something like this:

p1 = Person.by_last_name params[:last_name] if params[:last_name]
p2 = Person.by_age params[:age] if params[:age]
p3 = Person.by_sex params[:sex] if params[:sex]
 
people += p1 if p1.length + p2 + p3

What you really want is Ryan Bates’ Scope Builder gem.

Scope Builder lets you “build up named scopes conditionally.”  From the README:

This gem adds the scope_builder method to all Active Record models. A
builder behaves exactly like any other named scope except that calling
other named scopes on it will alter the builder itself rather than
returning a new named scope.

Here’s some example code using that defines Person.search using named scopes for last_name, age, and sex:

class Person < ActiveRecord::Base
  named_scope :by_last_name, lambda { |last_name|
    {
      :conditions => ['last_name = :last_name',
        { :last_name => last_name } ]
    }
  } 
 
  named_scope :by_age, lambda { |age|
    {
      :conditions => ['age = :age',
        { :age => age } ]
    }
  } 
 
  named_scope :by_sex, lambda { |sex|
    {
      :conditions => ['sex = :sex',
        { :sex => sex } ]
    }
  }
 
  def self.search(options)
    scope_builder do |builder|
      builder.by_last_name(options[:last_name]) if options[:last_name]
      builder.by_age(options[:age]) if options[:age]
      builder.by_sex(options[:sex]) if options[:sex]
  end
end

So, now all you need is a People Search form and an action to handle finding the right people.  Here’s what your search action might look like inside PeopleController:

def search
  @people = Person.search(params).all
end

Important note: When using ScopeBuilder as shown above, you’ll have to call #all because, in this case, Person.search returns a ScopeBuilder object, not a collection of ActiveRecord Person objects.  To get at the people collection, you call #all on the ScopeBuilder object returned by Person.search.

So, see?  Isn’t that pretty?  You’ve got a simple search that will find all the people that meet any all of the attributes passed in from the search form.  Ta da!