xml - How to select element by attribute with xslt

Question

I have this xml file:

<writer id_writer="1">
  <name>name</name>
</writer>

<film id_writer="1">
</film>

How to print the writer of each film using xslt

<xsl:for-each select="film">
   <xsl:value-of select="writer[@id_writer='what to put here']/name"></xsl:value-of>
</xsl:for-each>

Answer 1

The most efficient way to resolve cross references like this in XSLT is generally to use a key. The key definition goes at the top level of your stylesheet, outside any templates, and specifies which nodes you want to retrieve and how to compute the identifying key value for each one

<xsl:key name="writerById" match="writer" use="@id_writer" />

Once the key is defined you can retrieve matching nodes by key value using a function

<xsl:value-of select="key('writerById', @id_writer)/name"/>

Some XSLT processors (notably recent versions of Saxon EE) will automatically optimise //writer[@id_writer = current()/@id_writer] predicates to execute as efficiently as the explicit key-based approach, but this is not something you can necessarily rely on if you are using an older or less sophisticated processor.