XPath descendant-or-self vs ancestor-or-self in predicate?-CodePudding

I have an XPath expression that is working the way I want it to work but I don't understand why it's working. Here is the sample XML:

<?xml version="1.0"?>
<PurchaseOrders>
  <PurchaseOrder PurchaseOrderNumber="99503" OrderDate="1999-10-20">
    <Address Type="Shipping">
      <Name>Ellen Adams</Name>
      <Street>123 Maple Street</Street>
      <City>Mill Valley</City>
      <State>CA</State>
      <Zip>10999</Zip>
      <Country>USA</Country>
    </Address>
    <Address Type="Billing">
      <Name>Tai Yee</Name>
      <Street>8 Oak Avenue</Street>
      <City>Old Town</City>
      <State>PA</State>
      <Zip>95819</Zip>
      <Country>USA</Country>
    </Address>
    <DeliveryNotes>Please leave packages in shed by driveway.</DeliveryNotes>
    <Items>
      <Item PartNumber="872-AA">
        <ProductName>Lawnmower</ProductName>
        <Quantity>1</Quantity>
        <USPrice>148.95</USPrice>
        <Comment>Confirm this is electric</Comment>
      </Item>
      <Item PartNumber="926-AA">
        <ProductName>Baby Monitor</ProductName>
        <Quantity>2</Quantity>
        <USPrice>39.98</USPrice>
        <ShipDate>1999-05-21</ShipDate>
      </Item>
    </Items>
  </PurchaseOrder>
  <PurchaseOrder PurchaseOrderNumber="99505" OrderDate="1999-10-22">
    <Address Type="Shipping">
      <Name>Cristian Osorio</Name>
      <Street>456 Main Street</Street>
      <City>Buffalo</City>
      <State>NY</State>
      <Zip>98112</Zip>
      <Country>USA</Country>
    </Address>
    <Address Type="Billing">
      <Name>Cristian Osorio</Name>
      <Street>456 Main Street</Street>
      <City>Buffalo</City>
      <State>NY</State>
      <Zip>98112</Zip>
      <Country>USA</Country>
    </Address>
    <DeliveryNotes>Please notify me before shipping.</DeliveryNotes>
    <Items>
      <Item PartNumber="456-NM">
        <ProductName>Power Supply</ProductName>
        <Quantity>1</Quantity>
        <USPrice>45.99</USPrice>
      </Item>
    </Items>
  </PurchaseOrder>
  <PurchaseOrder PurchaseOrderNumber="99504" OrderDate="1999-10-22">
    <Address Type="Shipping">
      <Name>Jessica Arnold</Name>
      <Street>4055 Madison Ave</Street>
      <City>Seattle</City>
      <State>WA</State>
      <Zip>98112</Zip>
      <Country>USA</Country>
    </Address>
    <Address Type="Billing">
      <Name>Jessica Arnold</Name>
      <Street>4055 Madison Ave</Street>
      <City>Buffalo</City>
      <State>NY</State>
      <Zip>98112</Zip>
      <Country>USA</Country>
    </Address>
    <Items>
      <Item PartNumber="898-AZ">
        <ProductName>Computer Keyboard</ProductName>
        <Quantity>1</Quantity>
        <USPrice>29.99</USPrice>
      </Item>
      <Item PartNumber="898-AM">
        <ProductName>Wireless Mouse</ProductName>
        <Quantity>1</Quantity>
        <USPrice>14.99</USPrice>
      </Item>
    </Items>
  </PurchaseOrder>
<PurchaseOrder1 PurchaseOrderNumber="99504" OrderDate="1999-10-22">
    <Address Type="Shipping">
      <Name>Jessica Arnold</Name>
      <Street>4055 Madison Ave</Street>
      <City>Seattle</City>
      <State>WA</State>
      <Zip>98112</Zip>
      <Country>USA</Country>
    </Address>
    <Address Type="Billing">
      <Name>Jessica Arnold</Name>
      <Street>4055 Madison Ave</Street>
      <City>Buffalo</City>
      <State>NY</State>
      <Zip>98112</Zip>
      <Country>USA</Country>
    </Address>
    <Items>
      <Item PartNumber="898-AZ">
        <ProductName>Computer Keyboard</ProductName>
        <Quantity>1</Quantity>
        <USPrice>29.99</USPrice>
      </Item>
      <Item PartNumber="898-AM">
        <ProductName>Wireless Mouse</ProductName>
        <Quantity>1</Quantity>
        <USPrice>14.99</USPrice>
      </Item>
    </Items>
<PurchaseOrder PurchaseOrderNumber="99504" OrderDate="1999-10-22">
    <Address Type="Shipping">
      <Name>Jessica Arnold</Name>
      <Street>4055 Madison Ave</Street>
      <City>Seattle</City>
      <State>WA</State>
      <Zip>98112</Zip>
      <Country>USA</Country>
    </Address>
    <Address Type="Billing">
      <Name>Jessica Arnold</Name>
      <Street>4055 Madison Ave</Street>
      <City>Buffalo</City>
      <State>NY</State>
      <Zip>98112</Zip>
      <Country>USA</Country>
    </Address>
    <Items>
      <Item PartNumber="898-AZ">
        <ProductName>Computer Keyboard</ProductName>
        <Quantity>1</Quantity>
        <USPrice>29.99</USPrice>
      </Item>
      <Item PartNumber="898-AM">
        <ProductName>Wireless Mouse</ProductName>
        <Quantity>1</Quantity>
        <USPrice>14.99</USPrice>
      </Item>
    </Items>
  </PurchaseOrder>
  </PurchaseOrder1>

</PurchaseOrders>

This is my XPath expression:

/PurchaseOrders/descendant-or-self::*[@OrderDate="1999-10-22" and 
                                      not(ancestor-or-self::PurchaseOrder1)]

This gives me all purchase orders with an order date of 10/20/1999 except the one under PurchaseOrder1 which is exactly the result I want. But what confuses me is that I had to use ancestor-or-self instead of not(descendant-or-self::PurchaseOrder1) to get the results that I want. Wondering why that is since the PurchaseOrder that I want to exclude is a descendant of PurchaseOrder1 and not an ancestor? If I use descendant-or-self it actually includes the one under PurchaseOrder1.

Also, if anyone has suggestions on how to get the same results with a different expression, I'd appreciate that too.

CodePudding user response：

Only ancestor-or-self::PurchaseOrder1 works because it's within a predicate whose context nodes are going to be PurchaseOrder elements (with the specified @OrderDate). There are no PurchaseOrder elements at all that have PurchaseOrder1 descendants (only ancestors), so it's only the ancestor:: axis that can make the differentiation you seek.

If your actual XML is no more complicated than the shown XML, you can avoid the ancestor:: and descendant:: axes altogether with this simpler XPath,

/PurchaseOrders/PurchaseOrder[@OrderDate="1999-10-22"]

because all PurchaseOrder elements of interest are children of PurchaseOrders.

If there are elided levels of heritage above PurchaseOrder necessitating the ancestor:: or descendant:: axes, consider this simpler XPath:

//PurchaseOrder[@OrderDate="1999-10-22"][not(ancestor::PurchaseOrder1)]