integrity Error

The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: order_id - order.id]

I am getting above error if i goto the menu purchases -> products -> slecting the product ->click on  "More" select purchases -> it shows tree view, click on form view. Then it will open a form. Can any one tell me  use of this form?. I thought this is for purchase order. But there wan no supplier field to select, and it's giving above error. My actual requirement is to, add purchase order from product screen itself.