You can use asBag() in an OCL expression when you need an unordered collection that retains duplicate elements.
What asBag() returns
asBag() converts its receiver (the value or collection before ->asBag()) to a Bag. A Bag is an unordered collection in which the same value can occur more than once.
When you apply the operator to a single object, the result is a Bag containing that object. When you apply it to a collection, the result is a Bag and any duplicates already present in the collection remain present.
Use asBag() when duplicates matter
Use asBag() when a later collection expression must be able to retain multiple occurrences of the same value. Do not use it to remove duplicates.
For example, this expression turns the current object into a one-element Bag:
self->asBag()If self is a Department, the result is a Bag containing that Department.
This expression converts a collection reached through the Departments role into a Bag:
self.Departments->asBag()The result contains the Department objects from self.Departments. If the input collection contains the same Department more than once, the Bag retains those occurrences.
Bag compared with Set
| Collection type | Element order | Duplicate elements | When to use it |
|---|---|---|---|
| Bag | Unordered | Retained | You need collection operations to keep repeated values. |
| Set | Unordered | Removed | You need each value only once. |
For example, Set{'String', 'EmailAddress'} is a Set, so it cannot contain the same string twice. Converting that Set with ->asBag() changes the collection type to Bag, but it cannot recreate duplicates that the Set has already removed.
Important behavior
- A Bag is unordered. Do not use
asBag()when your expression depends on element order. asBag()preserves duplicates that exist in the input; it does not add duplicates.- Converting a Set to a Bag does not restore values previously removed by Set uniqueness.
- Use collection operators such as intersection with attention to their declared collection types and return values.
Finding operators
To inspect the operators available for a type in MDriven Designer, open the OCL Editor and type the class or expression. See Documentation:OCL General Operators for this workflow.