join table with datamapper orm

[eluser]Unknown[/eluser]
I have 3 tables

tags (id, name ...)

contents (id, name ...)

contents_tags (id, content_id, tag_id)

when i save 1 row into tags id and 1 row into contents, then i want save insert_id of both tables tags and contents into contents_tags, i don't know

help me !