fix(document): allow validation of moddelless documents on node side

Fix Automattic#8237, fix Automattic#8272, and improve DX by making ephemeral models similar
to normal ones

lib/index.js

@@ -23,6 +23,7 @@ const STATES = require('./connectionstate');
 const Types = require('./types');
 const Query = require('./query');
 const Model = require('./model');
+const BrowserDocument = require('./browserDocument');
 const Document = require('./document');
 const applyPlugins = require('./helpers/schema/applyPlugins');
 const get = require('./helpers/get');
@@ -396,6 +397,25 @@ Mongoose.prototype.pluralize = function(fn) {
   return _mongoose._pluralize;
 };
 
+
+Mongoose.prototype.ephemodel = function(schema, skipInit) {
+  let ephemodel = function Ephemodel(doc, fields, skipId) {
+    if (doc instanceof Schema) {
+      throw new TypeError('Argument to `Ephemodel` must be a POJO, ' +
+        '**not** a schema. Make sure you\'re calling `mongoose.ephemodel()`, not ' +
+        '`mongoose.Ephemodel()`.');
+    }
+    BrowserDocument.call(this, doc, this.schema, fields, skipId, skipInit);
+  };
+  ephemodel.__proto__ = BrowserDocument;
+
+  ephemodel.prototype = Object.create(BrowserDocument.prototype);
+  ephemodel.prototype.constructor = ephemodel;
+
+  ephemodel.prototype.schema = schema;
+  return ephemodel;
+}
+
 /**
  * Defines a model or retrieves it.
  *