Feature: Option to disable creation of initial connection

[titanism]

Prerequisites

• [X] I have written a descriptive issue title
• [X] I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

If you manage multiple custom connections, a typical pattern is to simply create new connections with createConnection(uri, options). Doing so, it adds by default to the mongoose singleton of mongoose.connections, so you can iterate over mongoose.connections to see properties such as readyState.

The default connection however is always present. The only way to delete it (regardless of custom Mongoose instances) is to manually call mongoose.connection.destroy();. This isn't such a good pattern, as a better option might be new Mongoose({ createInitialConnection: false }); or something. Adding this feature would require some refactoring though.

[titanism]

This might not actually be too hard, but we could be wrong:

  // default global options
  this.options = Object.assign({
    pluralization: true,
    autoIndex: true,
-    autoCreate: true
+    autoCreate: true,
+    createInitialConnection: false
  }, options);
+
+  if (this.options.createInitialConnection) {
+    const conn = this.createConnection(); // default connection
+    conn.models = this.models;
+  }

-  const conn = this.createConnection(); // default connection
-  conn.models = this.models;

[titanism]

There might also be another bug here we discovered, whereas id is not unique.

https://github.com/Automattic/mongoose/blob/9d7bf10606e567990969aabe130f16b896e2d29b/lib/connection.js#L72-L76

If you create multiple connections, then call .destroy() on them, and then create more connections, multiple of the connections will have the same id property because .length does not check for uniqueness.

cc @vkarpov15

[titanism]

Confirmed this is a bug!

const mongoose = require('mongoose');
mongoose.createConnection()
mongoose.createConnection()
mongoose.connections[0].destroy()
mongoose.createConnection()
mongoose.createConnection()
mongoose.createConnection()
mongoose.connections.map(c => c.id) // outputs: [ 1, 2, 3, 3, 4 ]

You can see 3 appears twice.