Use a pop-up menu instead of a drop-down list for the user field.
make sure the field with the popup menu is the asset.user_id, not the user.user_id field.
You don't really even need a relationship unless you are pulling extra data for calculations, etc.
That works as long as your user names are unique. If you get two people with identical names, you won't be able to select the second individual.
You can either enforce unique values on your user name field (And add a middle initial or some such to distinguish between same named individuals) or you can show both columns in your value list and add the field from the related table next to your drop down field. You can block access to the field with Field/Control | Behavior or (better) use access privilege settings to prevent editing the name field.
The tables are linked via integer primary/foreign key, which is unique.
deltatango, i tried again with pop up and i think i works after i removed the relationship - i can not edit anything in the field.
Thanks a lot
Roman, Phill brings up a good point, however -
If you only show the second field,
Record 1 - Joe Smith and
Record 2 - Joe Smith
Will only appear once on the value list. You should show both the record ID and the Name to prevent grouping of the second field.