Stubbing a mongoose exec method using Bluebird and sinon in node.js - node.js

Question

I have a service layer that calls a database layer where a query is run. I am trying to stub the data layer returning a value but I keep getting this error
TypeError: User.find(...).exec is not a function.
I am using the bluebird module and am also using sinon for stubbing
//Service Layer
var dbLayer = require('../dbLayer');
exports.getUsers = function (req, res) {
dbLayer.GetUsersThatNeedToBeApproved(req.body.domain)
.then(function (data) {
res.json(data);
})
.catch(function(error){
routesLogger.logError(req.session.user.role, "getUsers", error);
res.status(400).send({error: StatusCodes.DBERROR});
});
};
//DB Layer
module.exports.GetUsersThatNeedToBeApproved = function (domain) {
return new Promise.resolve(User.find({$and: [{'domain.upgradeRequest': true}, {'domain.access': constants.ACCESSPRIVATE}, {'domain.name': domain}]}).exec())
.then(function (data) {
return data;
}).catch(function (error) {
routesLogger.logError("userDAL", "GetUsersThatNeedToBeApproved", error);
return error;
});
This is my test case
describe("Suite to get users in portal that need to be approved",function(){
var request = null;
var response = httpMocks.createResponse();
it("Should get a list of all users that need to be approved for a specific domain",function(done){
request = {
session: {
user: {
domain:{
name:'domain'
},
email: 'testUser#test.com',
role: 'Admin'
}
}
};
var expectedFindResponse = [
{
email: "test#xxx.com",
role: "T",
domain:{
upgradeRequest:true,
access:'private',
name:'domain'
}
},
{
"email": "test1#xxx.com",
"role": "T",
domain:{
upgradeRequest:true,
access:'private',
name:'domain'
}
}
];
var find = sinon.stub(mongoose.Model, "find").returns(Promise.resolve(expectedFindResponse));
admin.getUsers(request, response);
var responseData = response._getData();
});
});