Fixing that CosmosDB Error=2: The index path corresponding to the specified order-by item is excluded.

This bug needs three things:

1. You're using Azure CosmosDB (I know, I don't like it too)
2. Have a Mongoose query with a sort option against a ..
3. Field that inside a sub document.

The query option in question is `{ sort: req.query.order || '-metaData.inserted_at' }`. The metaData.inserted_at field is just a date field. MetaData is just a plain object that has a couple of date fields tracking updates, deletes and such. So when you submit the query it spits out the Error=2 response.

CAUTION: Azure CosmosDB has a emulator isn't really helpful here. It will probably point you in a different direction. In my case, I was able to replicate the error and found a fix in where I 'unchecked' the Provision Throughput option in creating the database. That didn't solve the problem on the server.

In fixing this, you have two options:

1. No sorting in your code. 
2. Create the index

I went with option #2. A bit of a hassle. I tried CosmosDB as if it's a MongoDB equivalent.

db.getCollection('collectionName').getIndexes();

db.getCollection('collectionName').createIndex({'metaData.inserted_at':-1});

// shorter version
db.collectionName.createIndex({'metaData.inserted_at':-1});

It will be pain if you created an index that's wrong because you have to delete and create it again.

References:

• https://docs.mongodb.com/manual/tutorial/manage-indexes/#modify-an-index
• https://docs.microsoft.com/en-us/azure/cosmos-db/index-overview