[Version 3] What to do with missing belongsTo relationships

[frederikbosch]

During the upgrade I did find out some things.

1. There is code within Ember Data that is focussing on this topic.
2. That code did fire during the upgrade of this addon to 3.0-betaX for has many relationships, but not for belongsTo. I have no idea why it did not. As my own app was not having troubles by it, I was OK with it. At least, temporarily.
3. So let's accept that missing hasMany relationships will lead to a rejection when fetching the relationship.
4. What to do with belongsTo? Is there any convention within Ember Data? If not, what would be our best guess with regard to offline applications? Or maybe a setting?
5. First let's open an issue here that describes our thinking, then open issue at Ember Data for clarification.