How to mock a MongoDB collection of type IMongoCollection<T> to return some predefined data? - mongodb

Question

Below is my code:
Controller/Action:
[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Register(CustomerViewModel model, string returnUrl = null)
{
try
{
ViewData["ReturnUrl"] = returnUrl;
// when debugging the test, _dbContext.Customers throws exception
CustomerDoc existingCustomer = await _dbContext.Customers.Find(o => o.email == model.email).FirstOrDefaultAsync();
if (existingCustomer != null)
{
ModelState.AddModelError("Email", "email already used.");
}
// other checkings
if (!ModelState.IsValid)
{
return View(model);
}
// if model state is valid, do something here
}
catch (Exception ex)
{
return View(model);
}
return View(model);
}
And my unit test code is:
[Fact]
public async Task should_return_view_with_errors_when_email_already_exists()
{
IEnumerable<CustomerDoc> customers = new List<CustomerDoc>
{
new CustomerDoc
{
email = "test#test.com"
}
};
_dbContextMock.SetupAllProperties();
// below line is causing the error
_dbContextMock.Setup(c => c.Customers).Returns(() =>(IMongoCollection<CustomerDoc>)customers);
CustomerViewModel model = new CustomerViewModel
{
email = "test#test.com"
};
CreateController();
var result = await _controller.Register(model);
Assert.IsType<ViewResult>(result);
Assert.False(_controller.ModelState.IsValid);
Assert.True(_controller.ModelState.ContainsKey("Email"));
}
As you can see in my unit test code comment, I am trying to mock a IMongoCollection to return some data. But I am not able to do so because _dbContext.Customers is throwing exception.
How can I mock IMongoCollection to return some predefined data?
I am using
asp.net core 2.1.0
mongodb driver 2.7.0

Answer 1

You declare customers as a List:
IEnumerable<CustomerDoc> customers = new List<CustomerDoc>
but then try to cast it to IMongoCollection
() =>(IMongoCollection<CustomerDoc>)customers
There's two immediate directions (but both have further issues to deal with):
1) Just return the list without the cast
() => customers
but I can't see the type of c.Customers so I suspect this will just move the issue. I'll take a guess that it's IMongoCollection<CustomerDoc> which is why you're trying to do the cast in the first place? This is problematic as the .Returns would need to be be associated with a function performing the equivalent of c.Customer.Find(). Even so, it's probably better than the alternative.
2) Changing the customers variable to a type that implements IMongoCollection.
Option 1 feels like the way to go as option 2 forces you to start dealing with lots of logic that really shouldn't be relevant to this piece of code.