You can use existing in OCL when you need to determine whether a collection still contains at least one non-deleted object, including while changes are being made in the current transaction.

Syntax

collection->existing

collection is an OCL collection, commonly a role (association end) such as self.Orders.

Return value

existing returns a Boolean value:

Collection state	Result
The collection contains one or more objects that have not been deleted.	true
The collection is empty.	false
The collection contains only objects deleted during the current transaction.	false

Check whether related objects remain

Use existing on a role when your rule needs to know whether at least one related object remains valid.

For example, a Customer has an Orders role:

self.Orders->existing

The expression returns true when the customer has at least one non-deleted Order. It returns false when the customer has no orders, or when all of its orders have been deleted in the current transaction.

Use it with optional roles

An optional role can have no related object. Use existing when the relationship may be empty or when its object may have been deleted before the transaction is committed.

For example, if Customer has an optional PrimaryOrder role, evaluate whether that role still has a valid object:

self.PrimaryOrder->existing

This returns false if PrimaryOrder has not been set or if the related order has been deleted in the current transaction.

existing compared with exists

existing checks whether a non-deleted object is present. exists(condition) evaluates a condition for the elements in a collection. Choose the operator based on what your expression needs to answer.

Expression	Question answered	Example
collection->existing	Does the collection contain at least one non-deleted object?	self.Orders->existing
collection->exists(condition)	Does at least one collection element satisfy the condition?	o.Total > 1000)

Use existing for presence checks. Use exists(condition) when you must test a property or relationship of the collection elements.

For example, these expressions test different things:

-- At least one valid Order remains
self.Orders->existing

-- At least one Order has a total greater than 1000
self.Orders->exists(o | o.Total > 1000)

See also

• Documentation:OCL General Operators
• Documentation:OCL Boolean Operators
• Documentation:OCLOperators length