Single path-element alias in top-level selects

[deusaquilus]

All top-level selects should have a alias but in the case of a nested (i.e. embedded object) only the last element will be used.
For example:

case class Name(first: String, last: String)
case class Person(name: Name, age: Int)

run(query[Person])
// SELECT p.first, p.last, p.age FROM Person p

In situations where things are re-wrapped in tuples, this will be resolved based on the name of the innermost property name of the thing in which each field is coming from:

run(query[Person].map(p => (p.name.first, p.name.last, p.age))
// SELECT p.first AS _1, p.last AS _2, p.age AS _3 FROM Person p

This applies with nested objects that are mapped as well. For example if Name is nested within an output tuple, the "last path element" of both p.first and p.last will be what they are in the Name object:

run(query[Person].map(p => (p.name, p.age))
// SELECT p.first AS first, p.last AS last, p.age AS _2 FROM Person p
// (age is AS _2 because it is a value-level thing that gets wrapped into a tuple)
// since aliases are redundant it gets simplified to:
// SELECT p.first, p.last, p.age AS _2 FROM Person p

In subselects this gets a bit more complex because subselect aliases have to be unique (e.g. imagine if the Name property appeared twice) so in order to know which one is which we propagate the whole path from in the nested object. That means that on the top level this must be re-aliased back into the last path element:

run(query[Person].map(p => (p.name, p.age).nested)
// SELECT p.namefirst AS FIRST, p.namelast as LAST, p.age AS _2 FROM (SELECT x.first AS namefirst, x.last AS namelast, x.age FROM Person p) AS p