General outline of expected functionality:

Reference fields should not be implied; they must be explicitly set in the model, complete with the collection they refer to
Each field should be able to contain n number of references
- MongoDB can handle this natively as an array
- Drivers for less flexible relational databases may need to abstract away a relations table

In the returned data, each reference should be expanded to the corresponding object
- There should be a hard limit on how many levels will be expanded (3? 10?)
- Also a way to detect and prevent recursive references
- By default, always return an array of objects. Add a single key for the model if the user wants to force a direct object
In filtering in URL patterns, only filtering by the stored IDs needs to be supported. CSV of IDs should be treated as an OR

If the posted value for the reference field is a string, or an array of strings, assume it's an object ID
- In strict mode, return an error if it's not a valid ID
If the posted value is a (JSON) object, or an array of objects, create a new entry based on the values
- Need to guard against a POST request with multiple reference fields, where the first one succeeds, but the second one fails validation; this should not result in a situation where the overall request fails, but that one straggler entry is retained in the referenced collection
- Surface a config value for max number of records created on the fly via a reference field, with a sensible default
When modifying an existing record, overwrite any existing value with whatever is sent, if it's sent
- Dropped references should not result in deleted records

References #158

Description