mongoose.connection.collections.collection.drop() throws error every other time - node.js

Question

I am setting up testing using Jest for an Node/Express/Mongo project. I have tried to write a function to clear collections so each test starts with a clean slate:
const clearCollection = (collectionName, done) => {
const collection = mongoose.connection.collections[collectionName]
collection.drop(err => {
if (err) throw new Error(err)
else done()
)
}
beforeEach(done => {
clearCollection('users', done)
})
And another try, with promises:
const clearCollection = collectionName => {
const collection = mongoose.connection.collections[collectionName]
return collection.drop()
}
beforeEach(async () => {
await clearCollection('users')
})
The problem is that it they both alternate between working and throwing an error. Every time I save the file, it either works perfectly, or throws an error, alternating each time. The errors are always either one of:
MongoError: cannot perform operation: a background operation is currently running for collection auth_test.users
MongoError: ns not found
I can get it to work 100% of the time (limited by the stack anyway) by making clearCollection() call itself inside a catch(), but this feels so wrong:
const clearCollection = collectionName => {
const collection = mongoose.connection.collections[collectionName]
return collection.drop()
.catch(() => clearCollection(collectionName))
}

Answer 1

I don't know why mongoose.connection.collections.<collection>.drop() randomly throws errors, but there is a simple way to remove all the documents in Mongoose, which works just fine for resetting the collection before tests:
beforeAll(async () => {
await User.remove({})
})
Works every time.

Answer 2

I was having a similar issue while trying to drop the database in the beginning of the tests and populating the database right after. In the first run, collections would be created; in the next one, I would get an error of 'database is in the process of being dropped.'; ...and it was alternating like this.
I was also using an "in memory" Mongodb, running $ run-rs -v 4.0.0 -s (https://www.npmjs.com/package/run-rs) in a separate terminal window before running my Mocha tests. Also, I have Mongoose 5.2.0 and Mocha 5.1.1 here.
I've found out that Mongoose not necessarily executes the the drop commands immediately. Instead, it will schedule them for when the connection is up.
So, there can be a race condition in which the promise of the drop command is resolved and you move on in the code until reaching the instruction for creating your collections... but the drop command didn't finish running yet, and you'll get the error for the creation the new collections. The drop finish running and for the next time you run your test, your database (or collection) is already dropped, and hence you'll be able to insert the new collections again.
So, this is how I've solved...
Run in the before hook:
test.dropDb = function(done) {
this.timeout(0)
let database = your_MongoDB_URI
mongoose.connect(database, function (err) {
if (err) throw err;
mongoose.connection.db.dropDatabase(function(err, result){
console.log('database dropping was scheduled');
});
})
mongoose.connection.on('connected', function() {
setTimeout(function(){ done() }, 3000);
});
}
Run in an nested before hook
test.createDb = function(done) {
this.timeout(0)
let createDb = require('./create-db.js')
createDb() // here I call all my MyCollections.insertMany(myData)...
.then( function() {
done()
})
.catch( function(err) {
assert( null == err)
});
}
Run in the after hook
test.afterAll = function(done) {
mongoose.connection.close()
.then( function() {
done()
})
}
I don't know why I had to explicitly close the connection in the after hook. I guess it has something to do with the way run-rs is programmed, but I didn't look into it. I just know that, in my case, closing the connection after the test was mandatory to get the expected behavior.