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What are foreign keys used for?
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Foreign keys that you can create in the Application Builder prevent deletion 
of a record in the lookup table if a reference to it is stored in the lookup field. 

For example, when a foreign key is created on the "Customer" field for "Invoices" 
item, user won't be able to delete a customer in "Customers" catalog if a 
reference to it is stored in "Invoices".

The soft delete attribute of the lookup item must be set to false (see 
:doc:`Item Editor Dialog </admin/items/item_editor_dialog>`
) for the lookup field to appear in the 
:doc:`Foreign Keys Dialog </admin/items/foreign_keys_dialog>`
