fix(connection): re-run Model.init() if re-connecting after explicitly closing a connection

[vkarpov15]

Fix #12047

Summary

The Model.init() function is responsible for:

1. Calling createCollection() to ensure the Model's underlying collection exists, unless autoCreate = false
2. Calling createIndexes() to ensure the indexes defined in the Model's schema exist, unless autoIndex = false

Mongoose calls Model.init() when you create a model using mongoose.model() or Collection.prototype.model(). The createCollection() and createIndexes() calls are queued up until Mongoose connects to MongoDB.

The issue is that, if you close a connection and then reconnect to a different database or cluster, Mongoose won't re-run init(). Because 1) Mongoose only runs init() when you call model(), and 2) Mongoose stores a $init property on a model that contains a promise wrapping the init() call here. If $init is set, then calling init() just returns $init. This is to avoid duplicate init() calls.

This fix deletes $init when the user calls Connection.prototype.close(), and ensures that Mongoose calls init() when the user opens a connection using openUri() or setClient(). So close() clears $init caches, and re-opening the connection means Mongoose re-runs init().

Related issue: #11916

Examples

See tests

[AbdelrahmanHafez]

LGTM, thanks 👍