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Robert Cemper · May 23, 2022

Once again I fail to follow the repeated almost religious secret of $LIST() encoding
that is celebrated by Support and Engineering over decades.
Especially when the use of the knowledge is labeled "illegal".
That's just disappointing.

The problem of a possible unexpected change rather indicates incomplete Release Notes to me. 

Robert Cemper · May 22, 2022

Do you expect to save #1) the input JSON object or
#2) a dump of the converted result?
For both:

set stream=##class( %Stream.GlobalCharacter).%New()

#1)  with myjson as your JSON input

do stream.Write(myjson)
do stream.%Save()

#2) dump the generated Object  obj  using my package Full-OBJ-Dump described here

do ##class(Z.obj).dumpToStream(.stream,obj,1)
do stream.%Save()  
Robert Cemper · May 17, 2022

Status= 1 justteslls you tht the HTTP connection was processed OK.
Next you need to take a look into your response object.

set res=Httprequest.HttpResponse
in terminal then ZW res to see in the Status is 200 OK
otherwise what you see I  res.Data might be just an error page.
Which is also OK for HTTP but not for your content.

• property ReasonPhrase as %String;

This is the human readable reason that goes with the StatusCode.

• property StatusCode as %Integer;

The HTTP status code. This is useful to determine if the request was successful. Look in the rfc for HTTP to see which codes are supported and what they mean. A human readable form of this code is stored as the ReasonPhrase

• property StatusLine as %String;

The HTTP status line. This is the first line of the response and signals if the request was successful or if there was a problem.
Robert Cemper · May 6, 2022

Now with the class definition available, I understand(?) what you are looking for.
I see 2 possible solutions: embedded SQL or an Index on Title
#1

ClassMethod TitleToRowId(title) As %String 
    [ PublicList = (title, rowid, SQLCODE) ]
{
  &SQL(
     SELECT RowId into :rowid 
     FROM REST.TITLE 
      WHERE Title = :title
      )
  if 'SQLCODE quit rowid
  quit SQLCODE
}
 

btw: SQLCODE=0 means success. 
and you get the RowId by 

SET RowId=##class(REST.TITLE).TitleToRowId(obj.Title)  ;obj=JSON
SET task.Title = ##class(RESTAPI.TITLE).%OpenId(RowId)
SET book.Title = ##class(RESTAPI.TITLE).%OpenId(RowId) ; recent example

#2
 creating an Index on Title in REST.TITLE.
 but you have all trouble on duplicates, max. string length on that index
 So I'd position it as elegant but rather risky on maintenance
 

Robert Cemper · May 6, 2022

Now, this gets clear.
With the keyword IDKEY you replaced the default ID naming it RowID.

To store it:

set book=##class(BOOK).%New()
set book.RowId=obj.ID     ;  from JSON obj
set book.Title=obj.Title    ;  from JSON obj
do book.%Save()

to retrieve an existing Rowid:

set book=##class(BOOK).%OpenId(obj.ID)   ;from JSON obj
,;; access or change your book.Title
Robert Cemper · May 5, 2022

I'm sorry. it seems you don't understand what I'm talking about.
You just gave me the names. Not the structure and definition. 

Expected example:

Class RestApi.Books Extends %Persistent
{
Property Title As %String;
Property Pages As %Integer; 

Storage Default
{
<Data name="BooksDefaultData">
<Value name="1">
<Value>%%CLASSNAME</Value>
</Value>
<Value name="2">
<Value>Title</Value>
</Value>
<Value name="3">
<Value>Pages</Value>
</Value>
</Data>
<DataLocation>^RestApi.BooksD</DataLocation>
<DefaultData>BooksDefaultData</DefaultData>
<IdLocation>^RestApi.BooksD</IdLocation>
<IndexLocation>^RestApi.BooksI</IndexLocation>
<StreamLocation>^RestApi.BooksS</StreamLocation>
}
}
  
 Class RestApi.Title Extends %Persistent
{
Property Title As Books;
Property Text As %String; 

Storage Default
{
<Data name="TitleDefaultData">
<Value name="1">
<Value>%%CLASSNAME</Value>
</Value>
<Value name="2">
<Value>Title</Value>
</Value>
<Value name="3">
<Value>Text</Value>
</Value>
</Data>
<DataLocation>^RestApi.TitleD</DataLocation>
<DefaultData>TitleDefaultData</DefaultData>
<IdLocation>^RestApi.TitleD</IdLocation>
<IndexLocation>^RestApi.TitleI</IndexLocation>
<StreamLocation>^RestApi.TitleS</StreamLocation>
}
}


it's midnight now. I finish 

maybe next week.