Updating Lookup Values with the REST API

Question

Is it possible to update a List Item's Lookup Field Value via the Rest API? I can retrieve the field value if I use $select and $expand, but I can't find documentation anywhere that explains how to update it.

Answer 1

Yes, you can. For each lookup field, the list has an implicit field that holds the id of the item in the target list. The name of this implicit field is the name of the lookup field plus Id. In Milind's answer, the lookup field is named State and the id of the item in the target list is 2. So, when you do the update using the REST API you would set StateId = 2.

var call = jQuery.ajax({
    url: _spPageContextInfo.webAbsoluteUrl +
        "/_api/Web/Lists/getByTitle('Sales Opportunities')/Items(5)",
    type: "POST",
    data: JSON.stringify({
        "__metadata": { type: "SP.Data.Sales_x0020_OpportunitiesListItem" },
        StateId: 2
    }),
    headers: {
        Accept: "application/json;odata=verbose",
        "Content-Type": "application/json;odata=verbose",
        "X-RequestDigest": jQuery("#__REQUESTDIGEST").val(),
        "IF-MATCH": item.__metadata.etag,
        "X-Http-Method": "PATCH"
    }
});

Answer 2

It depends whether Lookup field value is multiple or not, the following formats are used when updating field value via REST:

• Single Lookup value format: LookupFieldName: LookupIdVal
• Mutiple Lookup value format: LookupFieldName: {"results": [LookupIdVal1,LookupIdVal2] }

Examples

How to update single lookup value

var itemProperties = {
        "__metadata": { "type": 'SP.Data.TasksListItem' }, 
        "ProjectId": 1
    }; 
updateListItem('Tasks',4,itemProperties,
   function(){    
   },
   function(error){
    console.log(JSON.stringify(error));
   }
);

How to update multiple lookup value

var itemProperties = {
        "__metadata": { "type": 'SP.Data.TasksListItem' }, 
        "ProjectsId": {"results": [1,3] }
    }; 
updateListItem('Tasks',4,itemProperties,
   function(){    
   },
   function(error){
    console.log(JSON.stringify(error));
   }
);

where

function updateListItem(listName, itemId, itemProperties, success, failure) {

    $.ajax({
            url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('" + listName + "')/items(" + itemId + ")",
            type: "POST",
            contentType: "application/json;odata=verbose",
            data: JSON.stringify(itemProperties),
            headers: {
                "Accept": "application/json;odata=verbose",
                "X-RequestDigest": $("#__REQUESTDIGEST").val(),
                "X-HTTP-Method": "MERGE",
                "If-Match": '*'
            },
            success: function (data) {
                success();
            },
            error: function (data) {
                failure(data);
            }
    });
}

References

Manipulating list items in SharePoint Hosted Apps using the REST API

Answer 3

In case you're using Power Automate, when you use Send an HTTP request to SharePoint action, field name + Id is not working. Use this syntax instead:

{
    "lookup_field": {
        "Id": <id>
    }
}

Answer 4

You can use SPServices' UpdateListItems method, like below:

$().SPServices({
   operation: "UpdateListItems",
   async: false,
   debug:true,
   listName: "Sales Opportunities",
   ID: 5,
   valuepairs: [["State", "2;#Rhode Island"]],
   completefunc: function (xData, Status) {
      alert(xData.responseText);
   }
});