Updating foregn Key in Linq to sql

I have a simple 2 table relationship:

table 'a' contains FK to table 'b'. Table b contains a fixed number of records. A record in table 'a' over time need to point to different

records in table 'b'. I understand that in linq I cannot just chenge FK, I need to change the entity that it represents. So I do

a.b_entity = db.bs.single(b => b.id == <other id> );

<other_id> exists in database. It works exept for one minor thing, For some reason, LINQ replicated record in table b and assigns the id

of the new record. I do not need to create a new record! I need Fk in 'a' to piont directly to the record I selected in b and not its copy.

What am I doing wrong?