fix(document): allow setters to modify this when overwriting single…

… nested subdoc

Fix #7585

lib/document.js

@@ -1014,10 +1014,25 @@ Document.prototype.$set = function $set(path, val, type, options) {
     // a single nested doc. That's to make sure we get the correct context.
     // Otherwise we would double-call the setter, see gh-7196.
     if (this.schema.singleNestedPaths[path] == null) {
-      const setterContext = constructing && this.$__.$options.priorDoc ?
-        this.$__.$options.priorDoc :
-        this;
-      val = schema.applySetters(val, setterContext, false, priorVal);
+      // Init the new subdoc to the previous values of the doc, so
+      // getters/setters see the correct current state. We pass the new subdoc
+      // instead of the old subdoc because otherwise side effects in setters
+      // wouldn't work, see gh-7585
+      if (constructing && this.$__.$options.priorDoc) {
+        const priorVal = Object.assign({}, this.$__.$options.priorDoc._doc);
+        delete priorVal[this.schema.options.discriminatorKey];
+        init(this, priorVal, this._doc);
+      }
+
+      val = schema.applySetters(val, this, false, priorVal);
+
+      if (constructing && this.$__.$options.priorDoc) {
+        // Clear init-ed paths afterwards, because those should be paths that
+        // were in the previous doc and should not be in the new one.
+        for (const path of Object.keys(this.$__.activePaths.states.init)) {
+          delete this._doc[path];
+        }
+      }
     }
 
     if (!didPopulate && this.$__.populated) {