Resolve fields of objects lazily (#1849)

* Resolve object fields lazily

* Maybe resolve Map schemas lazily

* Fix Scala 3

* Cleanups

core/src/main/scala-2/caliban/schema/SchemaDerivation.scala

@@ -85,14 +85,22 @@ trait CommonSchemaDerivation[R] {
           Some(ctx.typeName.full)
         )
 
+    override private[schema] def resolveFieldLazily: Boolean = !ctx.isObject
+
     override def resolve(value: T): Step[R] =
       if (ctx.isObject) PureStep(EnumValue(getName(ctx)))
       else if ((ctx.isValueClass || isValueType(ctx)) && ctx.parameters.nonEmpty) {
         val head = ctx.parameters.head
         head.typeclass.resolve(head.dereference(value))
       } else {
         val fields = Map.newBuilder[String, Step[R]]
-        ctx.parameters.foreach(p => fields += getName(p) -> p.typeclass.resolve(p.dereference(value)))
+        ctx.parameters.foreach(p =>
+          fields += getName(p) -> {
+            lazy val step = p.typeclass.resolve(p.dereference(value))
+            if (p.typeclass.resolveFieldLazily) FunctionStep(_ => step)
+            else step
+          }
+        )
         ObjectStep(getName(ctx), fields.result())
       }
   }

core/src/main/scala-3/caliban/schema/SchemaDerivation.scala

@@ -4,7 +4,7 @@ import caliban.Value.EnumValue
 import caliban.introspection.adt.*
 import caliban.parsing.adt.Directive
 import caliban.schema.Annotations.*
-import caliban.schema.Step.ObjectStep
+import caliban.schema.Step.{ FunctionStep, ObjectStep }
 import caliban.schema.Types.*
 import caliban.schema.macros.{ Macros, TypeInfo }
 
@@ -154,6 +154,8 @@ trait CommonSchemaDerivation {
       else if (isInput) mkInputObject[R](annotations, fields, info, paramAnnotations)(isInput, isSubscription)
       else mkObject[R](annotations, fields, info, paramAnnotations)(isInput, isSubscription)
 
+    override private[schema] lazy val resolveFieldLazily: Boolean = fields.nonEmpty
+
     def resolve(value: A): Step[R] =
       if (fields.isEmpty) PureStep(EnumValue(name))
       else if (isValueType) {
@@ -163,9 +165,11 @@ trait CommonSchemaDerivation {
         val fieldsBuilder = Map.newBuilder[String, Step[R]]
         fields.foreach { case (label, _, schema, index) =>
           val fieldAnnotations = paramAnnotations.getOrElse(label, Nil)
-          fieldsBuilder += getName(fieldAnnotations, label) -> schema.resolve(
-            value.asInstanceOf[Product].productElement(index)
-          )
+          lazy val step        = schema.resolve(value.asInstanceOf[Product].productElement(index))
+          fieldsBuilder += getName(fieldAnnotations, label) -> {
+            if (schema.resolveFieldLazily) FunctionStep(_ => step)
+            else step
+          }
         }
         ObjectStep(name, fieldsBuilder.result())
       }

core/src/main/scala/caliban/schema/Schema.scala

@@ -81,6 +81,15 @@ trait Schema[-R, T] { self =>
    */
   def arguments: List[__InputValue] = Nil
 
+  /**
+   * Boolean flag indicating whether the [[resolve]] method should be wrapped in a [[FunctionStep]] when resolving
+   * object fields.
+   *
+   * Should be `false` except for objects or wrappers of objects. This prevents fields of case classes to be resolved when they're not
+   * requested in the query, which significantly improves performance in larger schemas
+   */
+  private[schema] def resolveFieldLazily: Boolean = false
+
   /**
    * Builds a new `Schema` of `A` from an existing `Schema` of `T` and a function from `A` to `T`.
    * @param f a function from `A` to `T`.
@@ -90,6 +99,7 @@ trait Schema[-R, T] { self =>
     override def arguments: List[__InputValue]                             = self.arguments
     override def toType(isInput: Boolean, isSubscription: Boolean): __Type = self.toType_(isInput, isSubscription)
     override def resolve(value: A): Step[R]                                = self.resolve(f(value))
+    override private[schema] def resolveFieldLazily: Boolean               = self.resolveFieldLazily
   }
 
   /**
@@ -110,7 +120,8 @@ trait Schema[-R, T] { self =>
       case _                                                         => true
     }
 
-    override def resolve(value: T): Step[R] =
+    override private[schema] def resolveFieldLazily: Boolean = self.resolveFieldLazily
+    override def resolve(value: T): Step[R]                  =
       self.resolve(value) match {
         case o @ ObjectStep(_, fields)  =>
           if (renameTypename) ObjectStep(name, fields) else o
@@ -174,7 +185,8 @@ trait GenericSchema[R] extends SchemaDerivation[R] with TemporalSchema {
 
       private lazy val fieldsForResolve = fields(false, false)
 
-      override def resolve(value: A): Step[R1] = {
+      override private[schema] def resolveFieldLazily: Boolean = true
+      override def resolve(value: A): Step[R1]                 = {
         val fieldsBuilder = Map.newBuilder[String, Step[R1]]
         fieldsForResolve.foreach { case (f, plan) => fieldsBuilder += f.name -> plan(value) }
         ObjectStep(name, fieldsBuilder.result())
@@ -282,7 +294,8 @@ trait GenericSchema[R] extends SchemaDerivation[R] with TemporalSchema {
     override def optional: Boolean                                         = true
     override def toType(isInput: Boolean, isSubscription: Boolean): __Type = ev.toType_(isInput, isSubscription)
 
-    override def resolve(value: Option[A]): Step[R0] =
+    override private[schema] def resolveFieldLazily: Boolean = ev.resolveFieldLazily
+    override def resolve(value: Option[A]): Step[R0]         =
       value match {
         case Some(value) => ev.resolve(value)
         case None        => NullStep
@@ -294,7 +307,8 @@ trait GenericSchema[R] extends SchemaDerivation[R] with TemporalSchema {
       (if (ev.optional) t else t.nonNull).list
     }
 
-    override def resolve(value: List[A]): Step[R0] = ListStep(value.map(ev.resolve))
+    override private[schema] def resolveFieldLazily: Boolean = ev.resolveFieldLazily
+    override def resolve(value: List[A]): Step[R0]           = ListStep(value.map(ev.resolve))
   }
   implicit def setSchema[R0, A](implicit ev: Schema[R0, A]): Schema[R0, Set[A]]                                        = listSchema[R0, A].contramap(_.toList)
   implicit def seqSchema[R0, A](implicit ev: Schema[R0, A]): Schema[R0, Seq[A]]                                        = listSchema[R0, A].contramap(_.toList)
@@ -379,6 +393,7 @@ trait GenericSchema[R] extends SchemaDerivation[R] with TemporalSchema {
       override def toType(isInput: Boolean, isSubscription: Boolean): __Type =
         kvSchema.toType_(isInput, isSubscription).nonNull.list
 
+      override private[schema] def resolveFieldLazily: Boolean = evA.resolveFieldLazily || evB.resolveFieldLazily
       override def resolve(value: Map[A, B]): Step[RA with RB] = ListStep(value.toList.map(kvSchema.resolve))
     }
   implicit def functionSchema[RA, RB, A, B](implicit