ACCESSING MULTIPLE DOCUMENTS WITH XSLT
XML Stylesheet Translations (XSLT) provide a robust set of functions 
for transforming one XML document into another. This is particularly 
useful when incoming or outgoing XML documents do not match the 
document type expected by the target system. There are many times when 
the target system might expect a single document despite the source 
data being distributed across multiple documents. We can solve this 
problem by using XSLT to transform multiple source documents into a 
single XML document.

A SAMPLE PROBLEM
We'll start by defining a simple scenario that we can use to illustrate 
the problem. Assume our customers are going to send us their orders 
using XML documents. We want to protect the price information in the 
XML documents because different customers have different negotiated 
prices, which can change often. Rather than put a per-item price in the 
XML, the customers will put the price code for the particular item. For 
simplicity, we'll say that there are five price codes being used based 
on the letters A through E. Each price code corresponds to the dollar 
amount listed in our proprietary Price.xml document, which we update 
regularly. Unfortunately, our order system doesn't understand 
Price.xml. Instead, it expects each line item in the order to have a 
real price value when input into the system.

THE ORDER
An example of the order XML document is shown below. This order 
contains two line items. The first item has a price code of E and the 
second has a price code of C.

<OrderInfo> 
  <Customer> 
    <AccountNumber>7723123</AccountNumber> 
  </Customer> 
  <LineItems> 
    <Item no=1> 
      <SKU>1234</SKU> 
      <Quantity>1</Quantity> 
      <PriceCode>E</PriceCode> 
    </Item> 
    <Item no=2> 
      <SKU>7723</SKU> 
      <Quantity>2</Quantity> 
      <PriceCode>C</PriceCode> 
    </Item> 
  </LineItems> 
</OrderInfo>  

Our price codes are kept in Price.xml, which is shown below.

<PriceCodes> 
  <Code> 
    <Letter>A</Letter> 
    <Price>10.00</Price> 
  <Code> 
<Code> 
    <Letter>B</Letter> 
    <Price>42.00</Price> 
  <Code> 
<Code> 
    <Letter>C</Letter> 
    <Price>6.75</Price> 
  <Code> 
<Code> 
    <Letter>D</Letter> 
    <Price>322.00</Price> 
  <Code> 
<Code> 
    <Letter>E</Letter> 
    <Price>15.50</Price> 
  <Code> 
</PriceCodes>

As the orders come in, we want to translate the <PriceCode> element 
into a <Price> element. We'll do this using XSLT's document() function 
and an XPath expression to extract the correct price code.

THE DOCUMENT() FUNCTION AND XPATH
Using the document() function, you can access multiple XML documents 
from within your XSLT template. You can open as many documents as you 
need. The basic process we'll use to open and access external documents 
is to assign the <PriceCodes> node of the Price.xml document to a 
variable inside the translation, and then use XPath to query that node 
for the particular price.

To open a document and assign it to a variable, use the following 
syntax in your XSLT template:

<xsl:variable name="prices" select= 
"document('Price.xml')/PriceCodes"/> 

This will open the Price.xml document in the current directory, find 
the <PriceCodes> node, and assign it to the prices variable. Now we can 
query this node to extract a price for a particular code using XPath. 
To translate the price code into the price, you would use something 
like the following:

<xsl:variable name="letter" select="PriceCode"> 
<xsl:value-of select= 
"$prices/Code[Letter=$letter]/Price"/> 

This extracts the value of the PriceCode from the input document into a 
variable called $letter. Then, it uses the $letter variable to identify 
a particular price using an XPath query. Below is a snippet of our XSLT 
template that will put all of this together.

<xsl:variable name="prices" select= 
"document('Price.xml')/PriceCodes"/>  
 
<xsl:template match="Item">  
<xsl:variable name="letter" select="PriceCode"> 
<Item> 
  <SKU><xsl:value-of select="SKU"></SKU> 
  <Quantity><xsl:value-of select= 
"Quantity"></Quantity> 
  <Price><xsl:value-of select= 
"$prices/Code[Letter=$letter]/Price"/> </Price> 
</Item> 
</xsl:template>

SUMMARY
Using XSLT templates to translate XML documents can be a very powerful 
way to move data from one system to another. Sometimes this can be 
challenging if the source data is in multiple documents. You can use 
XSLT's document() function to access the data in multiple XML documents 
to solve this problem.

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