my $q = SODA::Query->new();
  $q->constrain(isa => 'Person');
  $q->descend('age')->constrain(greater => 30);

  Person => (age > 30)

  +-------------+
  | SODA::Query |
  +-------------+     +------------------+
  | constraints |---->| SODA::Constraint |
  | sub-queries |--+  +------------------+
  +-------------+  |  | isa => 'Person'  |
                   |  +------------------+
                   |
                ('age')
                   |     +-------------+
                   +---->| SODA::Query |
                         +-------------+    +------------------+
                         | constraints |--->| SODA::Constraint |
                         | sub-queries |    +------------------+
                         +-------------+    | greater => 30    |     
                                            +------------------+
                                            
  my $q = SODA::Query->new();
  $q->constrain(isa => 'Person');
  $q->descend('age')
    ->constrain(greater => 30)
    ->and(
        $q->descend('age')
          ->constrain(smaller => 35)
  );

  Person => (age > 30) AND (age < 35)

  +-------------+
  | SODA::Query |
  +-------------+     +------------------+
  | constraints |---->| SODA::Constraint |
  | sub-queries |--+  +------------------+
  +-------------+  |  | isa => 'Person'  |
                   |  +------------------+
                   |
                ('age')
                   |     +-------------+
                   +---->| SODA::Query |
                         +-------------+      +------------------+
                         | constraints |--+-->| SODA::Constraint |
                         | sub-queries |  |   +------------------+
                         +-------------+  |   | greater => 30    |     
                                          |   |------------------|   
                                          |   | and              |--+
                                          |   +------------------+  |
                                          |                         |
                                          |   +------------------+  |
                                          +-->| SODA::Constraint |<-+  
                                              +------------------+
                                              | smaller => 35    |
                                              +------------------+   
                                              
  my $q = SODA::Query->new(); 
  $q->constrain(isa => 'Person');
  $q->descend('age')
    ->constrain(greater => 30)
    ->and(
        $q->descend('age')
          ->constrain(smaller => 35)
    )->or(
        $q->descend('age')
          ->constrain(equal => 55)    
  );
  
  Person => (age > 30) AND (age < 35) OR (age = 55)                                                                                                                                                                                                                      
                                            
  +-------------+
  | SODA::Query |
  +-------------+     +------------------+
  | constraints |---->| SODA::Constraint |
  | sub-queries |--+  +------------------+
  +-------------+  |  | isa => 'Person'  |
                   |  +------------------+
                   |
                ('age')
                   |     +-------------+
                   +---->| SODA::Query |
                         +-------------+      +------------------+
                         | constraints |--+-->| SODA::Constraint |
                         | sub-queries |  |   +------------------+
                         +-------------+  |   | greater => 30    |     
                                          |   |------------------|   
                                          |   | or               |-----+
                                          |   | and              |--+  |
                                          |   +------------------+  |  |
                                          |                         |  |
                                          |   +------------------+  |  |
                                          +-->| SODA::Constraint |<-+  |
                                          |   +------------------+     |
                                          |   | smaller => 35    |     |
                                          |   +------------------+     |                                                    
                                          |                            |
                                          |   +------------------+     |                                        
                                          +-->| SODA::Constraint |<----+
                                              +------------------+     
                                              | equal => 55      |  
                                              +------------------+   
                                              
  my $q = SODA::Query->new(); 
  $q->constrain(isa => 'Person');
  $q->descend('age')
    ->constrain(greater => 30)
    ->and(
        $q->descend('age')
          ->constrain(smaller => 35)
          ->or(
             $q->descend('age')
               ->constrain(equal => 55)    
          )
  );
  
  Person => (age > 30) AND ((age < 35) OR (age = 55)) 
  
  +-------------+
  | SODA::Query |
  +-------------+     +------------------+
  | constraints |---->| SODA::Constraint |
  | sub-queries |--+  +------------------+
  +-------------+  |  | isa => 'Person'  |
                   |  +------------------+
                   |
                ('age')
                   |     +-------------+
                   +---->| SODA::Query |
                         +-------------+      +------------------+
                         | constraints |--+-->| SODA::Constraint |
                         | sub-queries |  |   +------------------+
                         +-------------+  |   | greater => 30    |     
                                          |   |------------------|   
                                          |   | and              |--+
                                          |   +------------------+  |  
                                          |                         |  
                                          |   +------------------+  |  
                                          +-->| SODA::Constraint |<-+  
                                          |   +------------------+     
                                          |   | smaller => 35    |     
                                          |   |------------------|     
                                          |   | or               |--+
                                          |   +------------------+  |                                                    
                                          |                         |
                                          |   +------------------+  |                                        
                                          +-->| SODA::Constraint |<-+
                                              +------------------+     
                                              | equal => 55      |  
                                              +------------------+   
                                                
  my $q = SODA::Query->new();
  $q->constrain(instance_of => 'Session');
  $q->descend('user')
      ->descend('user_name')
      ->constrain(equal => 'stvn');

  Session => (Session.user.user_name == 'stvn')  

  +-------------+
  | SODA::Query |
  +-------------+     +------------------+
  | constraints |---->| SODA::Constraint |
  | sub-queries |--+  +------------------+
  +-------------+  |  | isa => 'Session' |
                   |  +------------------+
                   |
               ('user')
                   |     +-------------+
                   +---->| SODA::Query |
                         +-------------+
                         | constraints |     
                         | sub-queries |--+
                         +-------------+  |
                                          |
                                    ('user_name')
                                          |
                                          |   +-------------+
                                          +-->| SODA::Query |
                                              +-------------+     +------------------+ 
                                              | constraints |---->| SODA::Constraint |
                                              | sub-queries |     +------------------+
                                              +-------------+     | equal => 'stvn'  |
                                                                  +------------------+

  my $q = SODA::Query->new();
  $q->constrain(isa => 'Person');
  $q->descend('age')->constrain(greater => 30);
  
  my $q2 = SODA::Query->new();
  $q2->constrain(isa => 'Address');
  $q2->descend('zipcode')->constrain(equal => 10018);  

  $q->join(address_id => $q2);

  Person, Address => (Person.addresss_id == Address.address_id) AND 
                     ((Person.age > 30) AND (Address.zipcode = 10018))

  +-------------+
  | SODA::Query |
  +-------------+             +------------------+
  | constraints |------------>| SODA::Constraint |
  | sub-queries |--------+    +------------------+
  |-------------|        |    | isa => 'Person'  |
  | joins       |--+     |    +------------------+
  +-------------+  |     |
                   |  ('age')
                   |     |     +-------------+
                   |     +---->| SODA::Query |
                   |           +-------------+    +------------------+
             ('address_id')    | constraints |--->| SODA::Constraint |
                   |           | sub-queries |    +------------------+
                   |           +-------------+    | greater => 30    |     
                   V                              +------------------+       
            +-------------+
            | SODA::Query |
            +-------------+     +------------------+
            | constraints |---->| SODA::Constraint |
            | sub-queries |--+  +------------------+
            +-------------+  |  | isa => 'Address' |
                             |  +------------------+
                             |
                        ('zipcode')
                             |     +-------------+
                             +---->| SODA::Query |
                                   +-------------+    +------------------+
                                   | constraints |--->| SODA::Constraint |
                                   | sub-queries |    +------------------+
                                   +-------------+    | equal => 10018   |     
                                                      +------------------+