Add support for sorting joined tables

Sources/FluentMongoDriver/DatabaseQuery+Mongo.swift

@@ -2,17 +2,11 @@ import FluentKit
 import MongoKitten
 
 extension DatabaseQuery {
-    internal func makeMongoDBSort() throws -> MongoKitten.Sort? {
+    internal func makeMongoDBSort(aggregate: Bool) throws -> MongoKitten.Sort? {
         var sortSpec = [(String, SortOrder)]()
 
         for sort in sorts {
-            switch sort {
-            case .sort(let field, let direction):
-                let path = try field.makeMongoPath()
-                try sortSpec.append((path, direction.makeMongoDirection()))
-            case .custom:
-                throw FluentMongoError.unsupportedCustomSort
-            }
+            sortSpec.append(try sort.makeMongoDBSort(aggregate: aggregate))
         }
 
         if sortSpec.isEmpty {

Sources/FluentMongoDriver/MongoDB+Join.swift

@@ -123,6 +123,10 @@ extension DatabaseQuery {
             stages.append(match(filter))
         }
 
+        if let mongoDBSort = try makeMongoDBSort(aggregate: true) {
+            stages.append(sort(mongoDBSort))
+        }
+
         switch offsets.first {
         case .count(let offset):
             stages.append(skip(offset))

Sources/FluentMongoDriver/MongoDB+Read.swift

@@ -37,7 +37,7 @@ extension FluentMongoDatabase {
             }
             find.command.projection = projection.document
 
-            find.command.sort = try query.makeMongoDBSort()?.document
+            find.command.sort = try query.makeMongoDBSort(aggregate: false)?.document
 
             logger.debug("fluent-mongo find command=\(find.command)")
             let decoder = BSONDecoder()

Sources/FluentMongoDriver/Sort.swift

@@ -15,3 +15,22 @@ extension DatabaseQuery.Sort.Direction {
         }
     }
 }
+
+extension DatabaseQuery.Sort {
+    internal func makeMongoDBSort(aggregate: Bool) throws -> (String, SortOrder) {
+        switch self {
+        case .sort(let field, let direction):
+            let path: String
+
+            if aggregate {
+                path = try field.makeProjectedMongoPath()
+            } else {
+                path = try field.makeMongoPath()
+            }
+
+            return try (path, direction.makeMongoDirection())
+        case .custom:
+            throw FluentMongoError.unsupportedCustomSort
+        }
+    }
+}