NHibernate: mapping a dictionary of lists

You can't directly map it. There are two rules to consider:

• Always use interfaces for collections (eg. IList<T>, IDictionary<K,V>)
• NH does not support nested collections. I've never seen an application for it before and never heard someone requesting it.

Put your list of string into a class and use interfaces:

class StringList
{
  IList<string> Strings { get; private set; }
}

class Entity
{
  private IDictionary<string, StringList> stringDict;
}

You might even see some advantages of having such a class.

Mapping:

<class name="Entity">
  ...
  <map name="stringDict" table="Entity_StringDict" access="field">
    <key column="Entity_FK"/>
    <index column="Key" type="System.String"/>
    <composite-element class="StringList">
      <bag name="Strings" table="Entity_StringDict_Strings">
        <key column="Entity_StringDict_FK"/>
        <element type="System.String" column="String"/>
      </bag>
    </composite-element>
  </map>
</class>

Maps to three Tables:

• Table Entity
• Table Entity_StringDict
  • Column Entity_FK
  • Column Key
• Table Entity_StringDict_Strings
  • Column Entity_StringDict_FK
  • Column String