Expressions in action payloads

[ralfhandl]

Use case

The history of department D15 (example data from [OData-Temporal]) shall be merged into D08. Assume we already have a function toTimeslices that converts Departments('D15')/history into the TimesliceWithPeriod format so that

GET ~/Departments('D15')/history/Temporal.toTimeslices()

returns

[{"Timeslice": {
   "From": "2010-01-01",
   "To": "2011-01-01",
   "Name": "Services",
   "Budget": 1100
}},
 {"Timeslice": {
   "From": "2011-01-01",
   "To": "9999-12-31",
   "Name": "Services",
   "Budget": 1700
}}]

If action parameters were specified in the URL, one could write:

POST ~/Departments('D08')/history/Temporal.Update(deltaTimeslices=@delta)
&@delta=$root/Departments('D15')/history/Temporal.toTimeslices()

But action parameters must be passed in the payload, and this does not allow aliases and expressions.

Suggestion

POST ~/Departments('D08')/history/Temporal.Update

{
 "deltaTimeslices@expressionUrl":
  "$root/Departments('D15')/history/Temporal.toTimeslices()"
}

Or, because in a structured payload, a structured expression fits better than an expression string that must be parsed, we borrow notation from CSDL:

POST ~/Departments('D08')/history/Temporal.Update

{
 "deltaTimeslices@expressionCsdl": {
  "$Function": "Temporal.toTimeslices",
  "$Apply": [
   {"$Path": "$root/Departments('D15')/history"}
  ]
 }
}

Imported from ODATA-1505

[HeikoTheissen]

Close only after #1943 is merged.

[ralfhandl]

General agreement for the direction, park it for a few weeks to evaluate syntax alternatives.