Dynamic / high order attribute values (e.g. an attribute being a sum of two other attributes)

[fgalan]

The idea is to define somehow (an special operation on the entity?) virtual attributes that are the result of doing an operation over other attributes in the same entity.

Some examples:

• A and B being numeric attributes, we define S as the sum of A and B. If we do a GET on the entity we will get:

GET /v2/entities/E/attrs

{
  "A": { "value": 10, "type": "Number" },
  "B": { "value": 20, "type": "Number" },
  "S": { "value": 30, "type": "Aggregated" },
}

• A and B being arrays, we define S as the intersection of A and B. If we do a GET on the entity we will get:

GET /v2/entities/E/attrs

{
  "A": { "value": [ 1, 2, 3, 4], "type": "StructuredValue" },
  "B": { "value": [3, 4, 5, 6], "type": "StructuredValue" },
  "S": { "value": [3, 4], "type": "Aggregated" },
}

The key point is that, from the point of view of the client doing the GET requests, this is completely transparent. I mean, S should behave as "regular" attributes A and B (e.g. from the point of view of q filtering, etc.).

Of course, not only for synchronous GETs. Notifications should work the same way.

From an implementation point of view, two basic alternatives should be explored:

• Take advantage of the MongoDB aggregation framework
• Implement the operation at CB code space

[fgalan]

Probably this issue could be achieved leveraging issue #4513