why 'PrimitiveValue' or 'StartObject' node was expected error

Question

I am using SharePoint Online:

I have a form that has a few multi-select choice fields. When I try to add an item, I get the following error:

"value: "A node of type 'StartArray' was read from the JSON reader when trying to read a value of a property; however, a 'PrimitiveValue' or 'StartObject' node was expected."

Here's the code and the fields that are multi-select.

 $.ajax({
url: fullUrl,
method: "POST",
data: JSON.stringify({
  '__metadata': { 'type': 'SP.Data.AuditItem' },
  'Register': that.register,
  'RiskRegister': that.nextIndex,
  'Reopen': that.formatDate(that.reopen),
  'RiskOrIssue': that.riskOrIssue,
  'Locations': that.locations,               //<---multi-select choice field
  'ProblemT': that.probTitle,
  'ProblemStatement': that.problemStatement,
  'TaskOwner': that.taskOwner,
  'RiskOwner': that.riskOwner,
  'Auditor': that.auditor    //<--- multi-select choice field
}),
headers: {
  "accept": "application/json;odata=verbose",
  "content-type": "application/json;odata=verbose",
  "X-RequestDigest": $("#__REQUESTDIGEST").val()
},
success: function(){
  alert("Item Added!");
},
error: function(data){
  console.log(data);
}   
});    

How can I solve this issue?

Thank you!

Answer 1

Multi-select choice field value needs to set with a collection, here is a code snippet for your reference:

   <script type="text/javascript">
       var locations = ['Locations1','Locations3'];
       var Auditors =  ['Auditor1','Auditor3'];
       var item = {  
                    "__metadata": {  
                        "type": 'SP.Data.MyListListItem'
                    },  
                       "Title":'Test',
                       "Locations": { '__metadata': { 'type' : 'Collection(Edm.String)'}, results: locations },
                       "Auditor": { '__metadata': { 'type' : 'Collection(Edm.String)'}, results: Auditors }


                };    
  $.ajax({  
                    url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('MyList')/items",  
                    method: "POST",  
                    contentType: "application/json;odata=verbose",  
                    data: JSON.stringify(item),  
                    async: false,  
                    headers: {  
                        "Accept": "application/json;odata=verbose",  
                        "X-RequestDigest": $("#__REQUESTDIGEST").val() 
                    },  
                    success: function(data) {  
                        alert('The Request has been successfully Added'); 
                    },  
                    error: function(jqXHR, textStatus, errorThrown) {  
                        console.log(jqXHR.responseText);
                        alert('Error');  
                    }  
                });  
</script>

Reference:

Update multiple choice field in sharepoint using rest api