Xml – XSD nested element

The key element is not supposed to go inside the sequence element. Instead, you put it as a child of the xs:element that defines ReferralCollection. The selector and field are child elements of the key element that define what is supposed to be unique. Both are XPath expressions. The selector value sets up the XPath context, and the field elements (there can be more than one) define the combination of values that are supposed to be unique. So in your case, the key element might look something like this:

<xs:key name="URL">
  <xs:selector xpath="."/>
  <xs:field xpath="rd:keyword"/>
</xs:key>

The selector here selects the element itself, that is, ReferralCollection. Then, in that context, the field element denotes the values of the keyword elements, which are thus constrained to be unique. The value of an element in XPath is its text content.

I haven't done much with key elements (or XML Schema in general), so some details above might not be correct, but the basic idea is what I explained, so it should be easy to fix any mistakes I made. Oh, also note that I used the rd prefix for your READOO namespace; in my experience using explicit prefixes is always best when dealing with XPath, so you need the appropriate xmlns:rd attribute in your schema to make this work.

EDIT: So it seems you want something like this:

<xs:element name="ReferralCollection">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="URL" maxOccurs="unbounded">
        <xs:complexType>
          <xs:sequence>
            <xs:element name="stem" type="xsd:string"/>
            <xs:element name="keyword" maxOccurs="unbounded">
            ...
            </xs:element>
          ...
  </xs:complexType>

  <xs:key name="URL">
    <xs:selector xpath="./URL"/>
    <xs:field xpath="stem"/>
  </xs:key>
</xs:element>

Is this closer to what you meant? It has URL elements, one or more, each containing a stem element followed by one or more keyword elements. The key constraint should make the contents of the stem elements unique.

You're getting the error because xsd:restriction is not allowed as a child of xsd:element. Try adding your xsd:restriction to an xsd:simpleType and then specifying that type in your xsd:element.

You could add the xsd:simpleType directly to the xsd:element, but since you're using the same restriction 3 times, it makes more sense to put it in a simpleType outside of the elements.

Here's an example. I named the simpleType "stackOverflowTest":

<xsd:schema elementFormDefault="qualified" version="1.0" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <xsd:element name="company_donation_request" type="company_donation_requestType" />
  <xsd:complexType name="company_donation_requestType">
    <xsd:sequence>
      <xsd:element name="order" type="orderType"></xsd:element> 
      <xsd:element name="donation" type="donationType"></xsd:element>
    </xsd:sequence>
  </xsd:complexType>
  <xsd:complexType name="donationType">
    <xsd:sequence>
      <xsd:element name="campaign_key" type="stackOverflowTest" minOccurs="1" maxOccurs="1"/>
      <xsd:element name="amount" type="xsd:decimal" minOccurs="1" maxOccurs="1"/>
    </xsd:sequence>
  </xsd:complexType>
  <xsd:complexType name="orderType">
    <xsd:sequence>
      <xsd:element name="id" type="stackOverflowTest" minOccurs="1" maxOccurs="1"/>      
      <xsd:element name="fulfillment_date" type="stackOverflowTest" minOccurs="1" maxOccurs="1"/>      
    </xsd:sequence>
  </xsd:complexType>
  <xsd:simpleType name="stackOverflowTest">
    <xsd:restriction base="xsd:string">
      <xsd:minLength value="2"/>
      <xsd:maxLength value="255"/>
    </xsd:restriction>  
  </xsd:simpleType>
</xsd:schema>

Hope this helps.