When testing code which uses Mongoose models, you often run into problems when the schemas are not defined. This code provides an idea on how to mock these models (in a `beforeEach`) and clear them later (in an `afterEach`)

mock-mongoose-model.js

"use strict";

var mongoose = require("mongoose");

var mockMongooseModel = {};
module.exports = mockMongooseModel;

/**
 * This function is useful for tests that require Mongoose models
 * loaded prior to loading the test file. This should be called in
 * the tests `beforeEach` prior to loading the file to test. The
 * `clearMongooseModels` function should be called in the `afterEach`.
 */
mockMongooseModel.loadMongooseModels = function() {
    try {
        mongoose.model("User");
    } catch (err) {
        mongoose.model("User", {});
    }
    try {
        mongoose.model("SomeOtherModel");
    } catch (err) {
        mongoose.model("SomeOtherModel", {});
    }
    // ...
};

/**
 * This mirrors the above `loadMongooseModels` to clean up what
 * was done, and should be called in the `afterEach` of tests.
 */
mockMongooseModel.clearMongooseModels = function() {
    mongoose.models = {};
    mongoose.modelSchemas = {};
};