An unexpected 'PrimitiveValue' node was found when reading from the JSON reader

Question

I am getting this error while trying to post data to SharePoint list. It has one lookup field PostCategory.

{"readyState":4,"responseText":"{\"error\":{\"code\":\"-1, Microsoft.SharePoint.Client.InvalidClientQueryException\",\"message\":{\"lang\":\"en-US\",\"value\":\"An unexpected 'PrimitiveValue' node was found when reading from the JSON reader. A 'StartObject' node was expected.\"}}}","responseJSON":{"error":{"code":"-1, Microsoft.SharePoint.Client.InvalidClientQueryException","message":{"lang":"en-US","value":"An unexpected 'PrimitiveValue' node was found when reading from the JSON reader. A 'StartObject' node was expected."}}},"status":400,"statusText":"error"}

I have created another lookup field but there I was able to update by rest, but not able to update to the previous field.

PostCategory is the exisitng field and I have added Newcategory field. Both are lookup field, but there is some difference as there is no results array in NewCategory. I think may be this is the reason I am not able to post data to PostCategory field.

    $.ajax({  
    url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/GetByTitle('Posts')/items(1)",  
    type: "POST",  
    headers: {  
        "accept": "application/json;odata=verbose",  
        "X-RequestDigest": $("#__REQUESTDIGEST").val(),  
        "content-Type": "application/json;odata=verbose",  
        "IF-MATCH": "*",  
        "X-HTTP-Method": "MERGE"  
    },  
    data: "{__metadata:{'type':'SP.Data.PostsListItem'},PostCategoryId: 4}",    
    success: function(data) {  
        console.log(data.d.results);  
    },  
    error: function(error) {  
        console.log(JSON.stringify(error));  
    }  
});  

Answer 1

If your column's internal name is PostCategory and list's internal name is Posts then try using below code as your data in POST call:

data: JSON.stringify({
        "__metadata": { type: "SP.Data.PostsListItem" },
        "PostCategoryId": 2
      })

You can get the exact internal name of your column by following this article: How to find the Internal name of columns in SharePoint Online?

Is multiple selection allowed on PostCategory?

Update:

For multiple selection field you need to pass an object with a results property, that contains an array of the IDs (number). Here is an example:

data: JSON.stringify({
        "__metadata": { type: "SP.Data.PostsListItem" },
        "PostCategoryId": { "results": [2, 4] }
      })