io.getquill.generic

Based on valueComputation and materializeQueryMeta from the old Quill This was around to flesh-out details of the outermost AST of a query based on the fields of the object T in Query[T] that the AST represents. For an example say we have something like this:

import io.getquill.ast.{ Ident => Id, Property => Prop, _ }
case class Person(name: String, age: Int)
query[Person].map(p => p) // or just query[Person]

That would turn into an AST that looks like this:

Map(EntityQuery("Person"), Id("p"), Id("p"))

This query needs to be turned into SELECT p.name, p.age from Person p, the problem is, before Quats, Quill did not actually know how to expand Ident("p") into SelectValue(p.name), SelectValue(p.age) (see SqlQuery.scala) since there was no type information. Therefore...

// We needed to convert something that looks like this:
query[Person].map(p => p) // i.e. Map(EntityQuery("Person"), Id("p"), Id("p"))

// Into something that looks like this:
query[Person].map(p => p).map(p => (p.name, p.age))
// i.e. Map(Map(EntityQuery("Person"), Ident("p"), Ident("p")), Tuple(Prop(Id("p"),"name"), Prop(Id("p"),"age")))

This makes it easier to translate the above information into the finalized form

SELECT p.name, p.age FROM (SELECT p.* from Person p) AS p

(Note that redudant map would typically be flattened out since it is extraneous and the inner SELECT would no longer be present)

Some special provisions were made for fields inside optional objects:

case class Address(street: String, zip: Int)
case class Person(name: String, address: Option[Address])
// This:
query[Person]
// Would become this:
query[Person].map(p => (p.name, p.address.map(_.street), p.address.map(_.zip)))

Now, since Quats were introduced into Quill since 3.6.0 (technically since 3.5.3), this step is not necessarily needed for query expansion since Ident("p") is now understood to expand into it's corresponding SelectValue fields so for queries, this stage could technically be elimiated. However, this logic is also useful for ActionMeta where we have something like this:

case class Person(name: String, age: Int)
// This:
query[Person].insert(Person("Joe", 44))
// Needs to be converted into this:
query[Person].insert(_.name -> "Joe", _.age -> 44)
// Which is actually:
EntityQuery("Person").insert(
 Assignment(Id("x1"), Prop(Id("x1"), "name"), Constant("Joe")),
 Assignment(Id("x1"), Prop(Id("x1"), "name"), Constant(44))
)

The fact that we know that Person expands into Prop(Id("p"),"name"), Prop(Id("p"),"age")) helps us compute the necessary assignments in the InsertUpdateMacro.

Elaboration can be different whether we are encoding or decoding because we could have decoders for certain things that we don't have encoders for and vice versa. That means that the potentially something encoded as a value would be decoded as a case-class or vice versa. Therefore, we need to differentiate whether elaboration is used on the encoding side or the decoding side.

Note that much of the implementation of anyValEncoder/anyValDecoder is a workaround for: https://github.com/lampepfl/dotty/issues/12179#issuecomment-826294510

Originally, the idea was to simply pass the self in LowPriorityImplicits directly into the macro that creates the AnyValEncoders. That way, the implementation would be as simple as:

 trait LowPriorityImplicits { self: EncodingDsl =>
   implicit inline def anyValEncoder[Cls <: AnyVal]: Encoder[Cls] =
     new MappedEncoderMaker[Encoder, Cls](self)
 }

Then, the MappedEncoderMaker could just internally call self.mappedEncoder(mapped, encoder) (where this self is the one that is passed in from the LowPriorityImplicits).

Unfortunately however, because of Dotty#12179, this would create an implicit encoder which would never be found. This created the need for the additional abstraction of AnyValEncoderContext and AnyValDecoderContext which would define makeMappedEncoder/makeMappedDecoder stub methods that the LowPriorityImplicits methods anyValEncoder/anyValDecoder could delegate the actual encoding/decoding work into. Hopefully when Dotty#12179 is resolved all of this convoluted logic can be removed and we can go back to the simpler implementation.