fix(document): copy atomics when setting document array to an existin…

…g document array

Fix #7472

lib/types/documentarray.js

@@ -44,12 +44,6 @@ function MongooseDocumentArray(values, path, doc) {
   // TODO: replace this with `new CoreMongooseArray().concat()` when we remove
   // support for node 4.x and 5.x, see https://i.imgur.com/UAAHk4S.png
   const arr = new CoreMongooseArray();
-  if (Array.isArray(values)) {
-    values.forEach(v => {
-      arr.push(v);
-    });
-  }
-  arr._path = path;
 
   const props = {
     isMongooseDocumentArray: true,
@@ -59,6 +53,16 @@ function MongooseDocumentArray(values, path, doc) {
     _handlers: void 0
   };
 
+  if (Array.isArray(values)) {
+    if (values instanceof CoreMongooseArray) {
+      props._atomics = Object.assign({}, values._atomics);
+    }
+    values.forEach(v => {
+      arr.push(v);
+    });
+  }
+  arr._path = path;
+
   // Values always have to be passed to the constructor to initialize, since
   // otherwise MongooseArray#push will mark the array as modified to the parent.
   const keysMA = Object.keys(MongooseArray.mixin);