Q:

Cosmos DB: How to reference a document in a separate collection using DocumentDB API

I am new to Azure Cosmos DB using the DocumentDB API.  I plan to model my data so that one document references another document.  This is pretty straight forward, as described in Modeling document data.  However, I also would like to separate the related documents into different collections (this decision is related to how the data are partitioned).
Edit 7/24/2017: In response to a comment wondering why I chose to use separate collections:  The reasoning for a separate collections mainly comes down to partition keys and read/write priorities.  Since a certain partition key is required to be present in ALL documents in the collection, it then makes sense to separate documents that the chosen partition key doesn't belong.  After much weighing of options, the partition key that I settled on was one that would optimize write speeds and evenly distribute my data across shards - but unfortunately it did not logically belong in my "Metadata" documents.  Since there is a one to gazillion relationship between metadata and measurements, I chose to use a reference to the metadata in the measurements instead of embedding.  And because metadata would rarely (or never) be appended to each measurement, I considered the expense of an additional round-trip to the DB a very low concern.
Since the reference is a "weak link" that is not verified by the database, is it possible and wise to store additional information, such as the collection name?  That is, instead of having just a string id, we may use a kind of path?
Metadata document in collection "Metadata":
{
  "id": "metadata1",
  ...
}

Measurement document in collection "Measurements":
{
  "id": "measurement1",
  "metadata-id" : "../Metadata/metadata1",
  ...
}

Then, when I parse the data in my application/script I know what collection and document to query.
Finally, I assume there are other/better ways to go about this and I welcome your suggests (e.g. underscores, not slashes; use a symbol to represent a collection, like $Metadata; etc).  Or, is my use of relations spanning collections a code smell?
Thank you!
Edit: To the downvoter, can you please explain your reasoning?  Is my question uninformed, unclear, or not useful?  Why?

A:

You're thinking about this the wrong way and incurring significantly more cost for an "optimization" that isn't necessary since you're billed at the per collection level. What you should be doing is picking a more generic partition key. Something like key or partitionKey. The tradeoff here is that you'll need to ensure in your client application that you populate this property on all of your documents (it may lead to a duplicated value, but ultimately that's okay). They you can continue to use the value of whatever you chose originally for your Measurements document and set something different for your Metadata documents.
I've written about this extensively in some other answers here and I believe it's one of the biggest misunderstandings about using Cosmos effectively and at scale. It doesn't help that in many Cosmos examples they talk about picking a partitionKey like deviceId or postal code which implies that you're dealing with homogeneous documents.
Please refer to this question that I answered regarding homogeneous vs heterogeneous in documentdb. The biggest argument for this pattern is the new addition of Graph APIs in Cosmos which necessitate having many different types of entities in a single collection and supports exactly the use case you're describing minus the extra collections. Obviously when dealing with heterogeneous types there isn't going to be a single property present on all documents that is appropriate for a partition key which is why you need to go generic.

