How to create 1:1 relationship in MS CRM ?

Hi,

Can anyone please tell how can I create 1:1 relationship in Ms CRM?

Example: I have two custom entity Project and Project Manager. If I have assigned a manager to a Project then same manager should not be assigned to some other project.

–> Project ML assigned to manager “A.Mishra” then A.Mishra should have only one Project (ML). If user to try to assign other projects to A.Mishra then it should throw an error like Manager is already assigned on a Project something like that.

Thanks in advance!!