NHibernate Many-To-Many Is Deleting All Associations Before Inserting

I have a Users table and a Networks table with a many-to-many relationship between them (a user may be long to multiple networks and a network may contain many users). The many-to-many relationship is held in a "UserNetworks" table that simply has two columns, UserId and NetworkId. My classes look like this: ``` public class User { public IList<Network> Networks {get; set;} } public class Network { public IList<Usre> Users {get; set;} } ``` The NHibernate mappings for these many-to-many collections looks like this: User.hbm.xml: ``` <bag name="Networks" table="UserNetworks" cascade="save-update" inverse="true"> <key column="UserId" /> <many-to-many class="Network" column="NetworkId" /> </bag> ``` Network.hbm.xml: ``` <bag name="Users" table="UserNetworks" cascade="save-update"> <key column="NetworkId" /> <many-to-many class="User" column="UserId" /> </bag> ``` In my code, I create an association between a user and a network like so: ``` user.Networks.Add(network); network.Users.Add(user); ``` I would expect the SQL run to simply perform one INSERT to the UserNetworks table. Instead, it executes a DELETE on the UserNetworks table with NetworkID = X, then proceeds to reinsert all the UserNetworks rows back in along with the new association. What am I doing wrong?