From 7bfa630a4f7dd06c439fa73568caa5491356a7bd Mon Sep 17 00:00:00 2001
From: Linus Bohle <linus.bohle@icloud.com>
Date: Fri, 4 Feb 2022 11:20:58 +0100
Subject: [PATCH] final touches on iteration #1 of compatibility warnings for
 aggregation stages vs. connection MongoDB version

---
 Sources/MongoKitten/Aggregate.swift           |  4 ++--
 Sources/MongoKitten/AggregateStage.swift      | 17 +----------------
 .../QueryPrimitives/Projection.swift          | 19 +++++++++++++++++++
 3 files changed, 22 insertions(+), 18 deletions(-)

diff --git a/Sources/MongoKitten/Aggregate.swift b/Sources/MongoKitten/Aggregate.swift
index c93b0b72..859adc3b 100644
--- a/Sources/MongoKitten/Aggregate.swift
+++ b/Sources/MongoKitten/Aggregate.swift
@@ -66,9 +66,9 @@ public struct AggregateBuilderPipeline: QueryCursor {
         
         #if DEBUG
         
-        let minimalVersionRequired = stages.compactMap { \.minimalVersionRequired }.max()
+        let minimalVersionRequired = stages.compactMap(\.minimalVersionRequired).max()
         
-        if let actualVersion = connection.wireVersion, let minimalVersion = minimalVersionRequired, actualVersion < minimalVersion {
+        if let actualVersion = await connection.wireVersion, let minimalVersion = minimalVersionRequired, actualVersion < minimalVersion {
             connection.logger.warning("Aggregation might fail since one or more aggregation stages require a higher MongoDB version than provided by the current connection.")
         }
         
diff --git a/Sources/MongoKitten/AggregateStage.swift b/Sources/MongoKitten/AggregateStage.swift
index 682cd40d..fad1bee1 100644
--- a/Sources/MongoKitten/AggregateStage.swift
+++ b/Sources/MongoKitten/AggregateStage.swift
@@ -39,24 +39,9 @@ public struct AggregateBuilderStage {
     }
     
     public static func project(_ projection: Projection) -> AggregateBuilderStage {
-        func valueUnwrapper(_ value: Primitive) -> WireVersion?{
-            switch value {
-            case let value as Int32:
-                (value == 0) ? return .mongo3_4 : return nil // indicates excluded fields
-            case let value as String:
-                (value == "$$REMOVE") ? return .mongo3_6 : return nil // indicates conditionally excluded fields
-            case let value as Document:
-                return valueUnwrapper(value)
-            default:
-                return nil
-            }
-        }
-        
-        let minimalVersionRequired: WireVersion? = projection.document.values.compactMap{ valueUnwrapper($0) }.min()
-        
         return AggregateBuilderStage(document: [
             "$project": projection.document
-        ], minimalVersion: minimalVersionRequired)
+        ], minimalVersion: projection.minimalVersion)
     }
     
     public static func project(_ fields: String...) -> AggregateBuilderStage {
diff --git a/Sources/MongoKittenCore/QueryPrimitives/Projection.swift b/Sources/MongoKittenCore/QueryPrimitives/Projection.swift
index ffd5be4a..8bf884b0 100644
--- a/Sources/MongoKittenCore/QueryPrimitives/Projection.swift
+++ b/Sources/MongoKittenCore/QueryPrimitives/Projection.swift
@@ -2,9 +2,28 @@ import BSON
 
 public struct Projection: Encodable, ExpressibleByDictionaryLiteral {
     public internal(set) var document: Document
+    public internal(set) var minimalVersion: WireVersion? {
+        func valueUnwrapper(_ value: Primitive) -> WireVersion?{
+            switch value {
+            case let value as Int32:
+                return value == 0 ? .mongo3_4 : nil // indicates excluded fields
+            case let value as String:
+                return value == "$$REMOVE" ? .mongo3_6 : nil // indicates conditionally excluded fields
+            case let value as Document:
+                return value.values.compactMap(valueUnwrapper).max()
+            default:
+                return nil
+            }
+        }
+        
+        return self.document.values.compactMap(valueUnwrapper).max()
+    }
 
     /// An expression that can be specified to either include or exclude a field (or some custom value)
     public enum ProjectionExpression: ExpressibleByBooleanLiteral, ExpressibleByStringLiteral, ExpressibleByDictionaryLiteral, PrimitiveConvertible {
+        /// Conditional exclusion of a field from the projection
+        public static let remove = ProjectionExpression.custom("$$REMOVE")
+        
         /// Creates a BSON.Primitive of this ProjectionExpression for easy embedding in Documents
         public func makePrimitive() -> Primitive? {
             switch self {