Converting a one-to-many xml design to a many-to-many

Question

I have an xml file format that looks something like this:

<topLevelTag>
  <category name="foo">
    <item attrib="value">ItemName</item>
    <item attrib="value2">AnotherItem</item>
  </category>
  <category name="bar">
    <item attrib="value">ItemName</item>
    <item attrib="value2">AnotherItem</item>
  </category>
</topLevelTag>

The code that parses this creates an instance of ItemName and is told that it's one category is, in the first case, "foo", and then creates a second instance of ItemName which has associated category "bar".

The problem is that I need to redesign the system so that each Item can have more than one Category. But I still need to be able to create multiple items as well. For example (using bullets instead of xml) I might need to create the following instances:

• ItemName
  • Primary Category - "foo"
  • Secondary Category - "bar"
• ItemName
  • Primary Category - "bar"
  • Secondary Category - "foo"
• ItemName
  • Primary Category - "foo"
  • Secondary Category - "quux"
• AnotherItem
  • Primary Category - "baz"
  • Secondary Category - "foo"
  • Tertiary Category - "monkey"
  • etc ... (no cap)
• StillAnotherItem
  • etc ...
• etc ... (no cap)

How can I design my XML format to encapsulate this many-to-many relationship?

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Answer 1

Why can't you use the same structure as in your bullets?

<topLevelTag>
    <item id="1">
        <value>ItemName</value>
        <category level="primary">foo</category>
        <category level="secondary">bar</category>
    </item>
    <item id="2">
        <value>ItemName</value>
        <category level="primary">foo</category>
        <category level="secondary">bar</category>
    </item>
</topLevelTag>

(the "id" distinguishes two instances of the same itemName).

For sure, you can't have a "many to many" structure explicitly - you must pivot (root?) either on item, or on category, since you need a hierarchical data set.

Or you change completely layout and pivot on the many-to-many relationship; this is easiest to convert to and from a RDBMS storage.

<topLevelTag>
    <item id="1">...</item>
    <category id="1">foo</category>
    <mapping>
        <map item="1" category="1" />
        <map item="1" category="2" />
        <map item="2" category="2" />
    </mapping>
</topLevelTag>