fix(document): run setter only once when doing .set() underneath a …

…single nested subdoc

Fix #7196

lib/document.js

@@ -980,10 +980,16 @@ Document.prototype.$set = function $set(path, val, type, options) {
       didPopulate = true;
     }
 
-    const setterContext = constructing && this.$__.$options.priorDoc ?
-      this.$__.$options.priorDoc :
-      this;
-    val = schema.applySetters(val, setterContext, false, priorVal);
+    // If this path is underneath a single nested schema, we'll call the setter
+    // later in `$__set()` because we don't take `_doc` when we iterate through
+    // a single nested doc. That's to make sure we get the correct context.
+    // Otherwise we would double-call the setter, see gh-7196.
+    if (this.schema.singleNestedPaths[path] == null) {
+      const setterContext = constructing && this.$__.$options.priorDoc ?
+        this.$__.$options.priorDoc :
+        this;
+      val = schema.applySetters(val, setterContext, false, priorVal);
+    }
 
     if (!didPopulate && this.$__.populated) {
       delete this.$__.populated[path];