Fixed test case to a real world scenario, and subsequently fixed type definition

types/mongoose-simple-random/index.d.ts

@@ -4,6 +4,12 @@
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 /// <reference types="mongoose" />
 
+declare module 'mongoose-simple-random' {
+  import mongoose = require('mongoose');
+  var _: (schema: mongoose.Schema) => void;
+  export = _;
+}
+
 declare module "mongoose" {
     interface Model<T extends Document> extends NodeJS.EventEmitter, ModelProperties {
         findRandom(conditions: Object, projection?: Object | null, options?: Object | null, callback?: (err: any, res: T[]) => void)

types/mongoose-simple-random/mongoose-simple-random-tests.ts

@@ -1,4 +1,5 @@
 import * as mongoose from 'mongoose';
+import * as mongoose_simple_random from "mongoose-simple-random";
 
 // test compatibility with other libraries - from @types/mongoose
 import * as _ from 'lodash';
@@ -13,3 +14,5 @@ mongoose.Model.findRandom({
 }, {}, { limit: 1 }, (error, data) => {
     if (error) { console.error("Error!"); } else { console.log("Success!"); }
 });
+
+mongoose.plugin(mongoose_simple_random);