Retrieve related fields with QUERY function on Custom Object

[Lindsay Klatzkin]

I have a custom object that has relationships to standard objects.  For example, my custom object has a relationship to Vendor, and I've added two related fields for it.

When I perform a readByQuery function, I am able to select these in the "select" parameter without error.

        <readByQuery>

          <object>offset_history</object>

          <fields>id,RVENDOR,vendor_name</fields>

          <query></query>

          <pagesize>100</pagesize>

        </readByQuery>

 

this successfully returns:

<data listtype="offset_history" count="8" totalcount="8" numremaining="0" resultId="">

                <offset_history>

                    <id>11954</id>

                    <RVENDOR>20062</RVENDOR>

                    <vendor_name>Paramount Consulting</vendor_name>

                </offset_history>

when I try to do the same thing using the QUERY function, I get an error

       <query>

        <object>offset_history</object>

        <select>

            <field>id</field>

            <field>RVENDOR</field>

            <field>vendor_name</field>

        </select>
       </query>

the error is:

                <error>

                    <errorno>XL03000006</errorno>

                    <description></description>

                    <description2>The following fields cannot be queried: [

    offset_history: [vendor_name] ] [Support ID: VCxwjWEB034%7EZvsZ4P5M0Pr4C8C-zu0_ywAAABA]</description2>

                    <correction></correction>

                </error>

I have tried replacing vendor_name with VENDOR.NAME and RVENDOR.NAME and it errors both times.

How can I use the QUERY function on a custom object with relationships?