Mocha test cases throw error as "is not a function" for loaded module when run all test cases together

I am using mocha to run my unit test cases for my nodejs code. However I got surprised when I found that one of the module doesn't load its dependent module when run all the test cases together, however it works perfectly fine when I run these test file separately. I did explore each files but couldn't why it creates problem when I try to run all test files? Following are details.

Server.js (This is the problematic file)

var constants = require("./../utilities/Constants");
var log = require('./../utilities/Logger');
var validator = require('validator');
var Utilities = require('./../../common/Utilities');
var documentOperationModule = require('./../persistence/DocumentOperation');

module.exports = {
    loadAppConfig: function (callback) {              
        //load application config from database and export as module.
        documentOperationModule.getAllDocuments(constants.APPLICATION_CONFIG_COLLECTION, "_id", -1, function (err, appConfigs) {
            if (!err) {
                global.appConfig = appConfigs[0];
                callback();
            }
            else
            {
                callback(new Error("Unable to load app config data error " + err));
            }
        });
    }
};

It throws following error when run all test cases together

TypeError: documentOperationModule.getAllDocuments is not a function

However it works perfectly file when i run each test case file separately.

Profile.test.js

var expect = require('chai').expect;
var sinon = require('sinon');
var Utilities = require('app-root-path').require('/src/common/Utilities');
var Workout = require('app-root-path').require('/src/server/modules/Workout');
var UnitTestUtils = require('app-root-path').require('/src-test/unit/utils/UnitTestUtils');
var constants = require('app-root-path').require('/src/server/utilities/Constants');
var Server = require('app-root-path').require('/src/server/modules/Server');
//Test suit for getUserSchedule()
describe("getUserSchedule()", function () {

    it("should  get user schedule", function (done) {        
        var userId = "sddfgsdgf";        
        //load global config first
        Server.loadAppConfig(function () {
            expect(global.appConfig.defaultroutine).to.be.not.undefined;
            //generate user schedule
            Workout.getUserSchedule(userId, function (err, schedule) {
                expect(schedule.sequences[0]).to.include("w");
                //exit from test
                done();
            });
        });
    });
});

DocumentOperation.js

//Load database connection
var DBModule = require('./../persistence/DBModule.js');
var constants = require("./../utilities/Constants");
var AppUtilities = require('./../utilities/AppUtilities');
var Server = require('./../modules/Server');
var Utilities = require('./../../common/Utilities');
var log = require('./../utilities/Logger');
var Crypto = require('./../modules/Crypto');
module.exports = {

 // Get all docuemnts for given collection.
    getAllDocuments: function (collection, sortby, order, callback) {
        //If sortby is not given then use _id as sort order
        if (Utilities.isEmpty(sortby)) {
            sortby = "_id";
        }
        //If order direction is not given then use old item to display first 
        if (Utilities.isEmpty(order)) {
            order = -1; //note, -1 means to display old item at last and +1 means to display old item at top
        } else {
            order = parseInt(order);
        }
        log.debug({collection: collection}, "Finally doing sort by " + sortby + " and order as " + order);
        var sortObj = {};
        sortObj[sortby] = order;
        //Filter deleted document if any
        DBModule.db.collection(collection).find({isDeleted: {$ne: true}}, {password: 0, temppassword: 0}).sort(sortObj).toArray(function (err, docuemnts) {

            if (!err) {
                /callback(null, docuemnts);

            } else {
                callback(err, null);
            }
        });
    }

};

I am surprised and confused bcz of difference in behavior when run one test file and when run all test file together.

Please help.