UpdateListItems won't let you reset field value to empty?

Question

I've recently discovered that the Lists.asmx ignores your attempts to reset a field's value to empty string or null with Lists.UpdateListItems. Not a very well documented limitation discovered by trial and error.

Can someone advise on the possibility to reset field values using FP RPC / OWSAPI from AJAXy pages? Or am I missing something in the OOTB web service functionality / CAML syntax? I mean without deploying a custom Web Service just for that little generic task?

Answer 1

I just did a test with SPServices where I set a Single line of text column to an empty string (""), which seemed to work just fine. This was with WSS 3.0 (the simplest test case, IMO).

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

This generates the call:

<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <UpdateListItems xmlns="http://schemas.microsoft.com/sharepoint/soap/">
      <listName>Sales Opportunities</listName>
      <updates>
        <Batch OnError="Continue">
          <Method ID="1" Cmd="Update">
            <Field Name="StateID"></Field>
            <Field Name="ID">5</Field>
          </Method>
        </Batch>
      </updates>
    </UpdateListItems>
  </soap:Body>
</soap:Envelope>

EDIT:

I just did the same thing for a Lookup column and a Person or Group column. The valuepairs line is:

valuepairs: [["State", ""], ["Sales_x0020_Rep", ""]],

Both columns were "emptied".

Answer 2

In the end, had to resort to writing a custom web service that also makes great bandwidth savings.