[question?] counterintuitive class method behaviour

[k4lizen]

When I write a query like this:

class Meow extends Struct {
  Field f;
  Meow() {
    f.getDeclaringType() = this
  }
  Field getField() { result = f }
}

from Meow m
select m, m.getField(), m.getField()

My mental model of this is: "Meow is essentially a (struct, struct's field) tuple. from Meow m will select all of those tuples. For each of those , select m, m.getField(), m.getField() will print their contents."

But that is actually not what happens, because you get results like this:

|                 m                 |                        col1                         |                        col2                         |
+-----------------------------------+-----------------------------------------------------+-----------------------------------------------------+
// [snip]
| port_io_ops                       | f_inb                                               | f_outb                                              |
// [snip]

Which means that the first m.getField() and the second m.getField() are operating on different Meow objects.

I've been debugging a bug in my query for hours now only to realize that this is what happens. Is this really intended? I read through a non-trivial amount of documentation and did not realize this, I feel like my whole mental model of how codeql works is shattering :S

(If it is really intended, could the documentation be updated to point this out more clearly?)

[k4lizen]

I encountered this in the following situation, this doesn't work:

class MyStuct extends Struct {
  MyFlow::PathNode source;
  MyFlow::PathNode sink;
  MyStuct() {
      MyFlow::flowPath(source, sink) and
      some_predicate(this, sink) and
  }
  MyFlow::PathNode getSink() { result = sink }
  MyFlow::PathNode getSource() { result = source }
}
from MyStuct s
select s.getSink().getNode(), s.getSource(), s.getSink(), "aaa"

, and instead you must do this:

class MyStuct extends Struct {
  MyFlow::PathNode source;
  MyFlow::PathNode sink;
  MyStuct() {
      MyFlow::flowPath(source, sink) and
      some_predicate(this, sink) and
  }
  MyFlow::PathNode getSink() { result = sink }
  MyFlow::PathNode getSource() { result = source }
}
from MyStuct s, MyFlow::PathNode source, MyFlow::PathNode sink
where
  source = s.getSource() and sink = s.getSink() 
select sink.getNode(), source, sink, "aaa"

But it's not even clear to my why in the first case the s.getSource() and s.getSink() are being taken from different objects in the select statement, but in the second case, in the where statement, the source and sink are indeed being taken from the same object.

What is the rule here?