Simple assignment not working (I must not understand...) about jsonata HOT 9 CLOSED

I see, I forgot that the transform part is in the context of the target so I needed to do:
msg~>|payload|{"originalInput":(input)}|

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How would I reference some other part of the json in the transform clause?
For example, if I have:

{
  "msg":{
    "payload":{
      "entity":"junk",
      "context":{
        "display_type":{}
      }
    }
  }
}

and I wanted to set the msg.payload.context.display_type to the value of msg.payload.entity

I am trying:
msg~>|payload.context.display_type|(msg.payload.entity)|
and even though (msg.payload.entity) resolves to "junk" the display_type remains {}

I think I need a way to reference anything in the transform clause but am not sure how to do this.

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Try addin "$$" in front of msg.payload.entity (to get to the root context):

(
$ ~> |msg.payload.context|{"display_type": $$.msg.payload.entity}|;
)

See http://try.jsonata.org/H1-5P8Dkf

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Based on a solution from here:
#70 (comment)

It seems by creating a variable, once can then reference that variable in the transform clause.

However, I think the expectation is the transform clause is an object, not a simple string or number. The following does work, but I'd prefer not to have to assign the variable:

(
$entityVal := {"display_type":(msg.payload.entity)};
msg~>|payload.context|$entityVal|
)

For example, this does not work:
msg~>|payload.context|{"display_type":(msg.payload.entity)}|

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Thanks @cray74 -- that works if I create the object in the clause.

Is there any way to allow the transform clause to evaluate to a simple type (string, int...) and still allow assignment to the transform target? or must we always reference an object?

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try wrapping your target in $string('your string'), or $number(9999) or any other function (actually returning an object, if I'm not mistaken).

I'm a total noob btw, so I might be totally off (but I've spent a few hours last weekend with very similar challenges, see http://try.jsonata.org/SkrU3nByf).

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According to the new documentation, the update expression (your "display_type") must evaluate to an object -- and it is evaluated relative to the location object or array (your "payload.context"). Since there is no way to reference parent objects, the "entity" expression needs the "$$." prefix, to be referenced starting at the root of the incoming data.

So this expression seems to work for me:
msg~>| payload.context | { "display_type": $$.msg.payload.entity } |
which in human-readable form is

return a transformed "msg" object, updating the "payload.context" object to have a "display_type" property value copied from the "msg.payload.entity" field

I don't understand why, but it's clear that Andrew does not expect the update expression to return a simple type (string, number, boolean). I can imagine that by requiring an object to be returned, the $merge() function logic can be used on the results. But if the update returns a string, why not just replace the location's value instead of trying to merge it?

I think this syntax describes the intent of the change more precisely, and in fewer characters:
msg~>| payload.context.display_type | $$.msg.payload.entity |

Unfortunately, using the branch 1.4 implementation, this syntax returns the error:

The insert/update clause of the transform expression must evaluate to an object: "junk"

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The reason this only works at the object level is because it needs a reference to the thing it is going to update. Take the following example:

{
  "msg": {
    "payload": {
      "originalInput":"oldValue",
      "input": {
        "text": "get weather for jfk"
      }
    }
  }
}

If the expression was:
msg ~> | payload.originalInput | 'newValue' |
the processor would evaluate the expression payload.originalInput and get the string "oldValue". Not a pointer to that string; just the string value. It will have lost the reference to the object that contained that value. So we need to specify that the object we want to modify is payload, and the updates that we want to make to that object is specified by the next part of the expression. I can't think of a way round this at the moment. It's a similar situation to the discussion, and my response in issue #83.

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I believe this is resolved as per last comment. Please re-open if not.

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