You can use general OCL operators to query objects and collections in your model; this page helps OCL authors discover which operators apply in the current expression context.
Object Constraint Language (OCL) is a declarative, side-effect-free query language used throughout MDriven for expressions such as constraints, derivations, and ViewModel expressions. An operator is an operation that evaluates a value, an object, or a collection and returns a result.
Find operators in the OCL Editor
Use the OCL Editor to inspect the operators available for the type you are currently working with.
- Open the OCL Editor for the expression you are editing.
- Enter a class or expression that produces the value you want to work with.
- Inspect the operators offered for that class or value type.
- Add an operator, then continue the expression with the result it returns.
For example, when an expression produces a collection of Car objects, the available collection operators can help you count, filter, sort, or select objects from that collection. When an expression produces a Boolean value, use Boolean operators to combine or negate conditions.
The OCL Editor interface displays available operators for your current type.
Choose the operator category
Operators depend on the value before the operator. Use the following pages for the operator category you need.
| When your expression works with | Use | Example purpose |
|---|---|---|
| An object or a property value | OCL Operators | Evaluate or compare a property on an object. |
| A Boolean condition | OCL Boolean Operators | Combine conditions with and, or, or not.
|
| A collection of objects | OCL Collection Operators | Filter a collection, count its members, sort it, or retrieve an item. |
| A complete expression or a constraint | OCL Expressions | Place operators in a valid expression for a derivation, ViewModel, or constraint. |
Common operator patterns
Compare values
Use comparison operators to make a condition evaluate to true or false. For example, an age rule can require a person to be at least 18:
self.age >= 18You can combine conditions. This example accepts either of two gender values:
self.gender = 'Male' or self.gender = 'Female'See Boolean operators for logical combinations and OCL Operators for the available general operators.
Work with collections
A collection expression can be followed by collection operators. For example, allinstances obtains the instances of a referenced class, and size returns how many objects are in the resulting collection. If there are four cars, applying size returns 4.
Use first or last when you need one end of a retrieved collection. Use an ordering operator before selecting an item when the intended result depends on a property value. For example, ordering cars by registration number before taking a subsequence makes the chosen subset depend on that ordering.
Use select to retain only the objects whose condition is true. A selection uses a loop variable, the name assigned to each object while the condition is evaluated:
Cars->select(oneCar | oneCar.registrationNumber = 'ABC123')Here, oneCar is the loop variable. The pipe character (|) separates that variable from the Boolean condition. For collection-specific syntax and examples, see OCL Collection Operators.
Common collection operators are summarized in the following table.
| Operators | Description |
|---|---|
| allInstances | All the objects of the class |
| allinstancesAtTime | All the currently loaded instances |
| allLoadedObjects | All objects currently in memory, excluding deleted objects |
| allStates | Meta information about available states in state machines the class may contain |
| allSubClasses | Meta information on all the sub-classes this class has |
| allSuperTypes | Meta information on all the super-classes – in inheritance order the class has |
| ASCII | The set of symbols and characters used to specify constraints on objects in software development. |
| associationEnds | Returns a collection of strings with association names for the class |
| associationEndsWithType | Returns a collection of strings with association names and type information for the class |
| asString | The string representation of the class – the asString operation is available on everything |
| asTaJson | Returns a json string defined by the ViewModel |
| atTime | Gives the read-only representation of a versioned object as it was at time X. |
| Attributes | Meta information about what attributes the class has |
| BigEndianUnicode | A data encoding format for Unicode characters |
| brokenConstraints | Returns a collection of the names of broken constraints for the class and object. |
| canAccess | Check ViewModels Access expression for root |
| Constraints | Meta information on what constraints the class has |
| count | Returns how many times the object is in the collection self. |
| Empty | Used to check if a collection or string is empty or not |
| emptylist | Returns an empty list typed to hold objects of the class |
| existing | The existing operator returns true if the object hasn't been deleted. |
| externalId | Returns the external ID for the object operated on. |
| format | Used to create formatted strings based on a given pattern. |
| IsDirty | Returns true if the objects have been changed since the last save. |
| IsNew | IsNew operator returns true if the object has never been saved to the database. |
| isDirtyMember | Called on a class attribute to check if this attribute has changes pending to be saved. |
| isNull | Returns true if the value is null (not assigned any value). |
| maxValue | Gets the maximum value in a collection of values |
| minValue | Gets the minimum value in a collection of values |
| nullValue | A typed null value |
| objectFromExternalId | An external identity will be resolved to the object |
| oclAsType | The type of the class |
| oclGetStates | Gets a list of the objects all state machines currently state as strings. |
| oclIsInvalid | Returns true if self is equal to *invalid*. |
| oclIsKindOf | This is to if a class is a subclass or as the class itself and not unrelated |
| oclIsTypeOf | Returns true if |
| OnCreate | Is often called when a new object has been created. |
| OnUpdate | Is called just before the object is saved to the database. |
| oclIsUndefined | Returns true if self is equal to invalid or null. |
| OclType | When used on an object, you get the type of the object. |
| random | Used to generate a random value within a specified range or collection |
| safeCast | Used to safely cast an object to a subtype. |
| sqlpassthrough | Calls a stored procedure |
| sqlpassthroughobjects | Return a primary key of SomeClass. This can be combined with other Ocl-PS via ->intersection or union. |
| superTypes | Returns the set of all direct supertypes of the type. |
| TaggedValue | Meta information on tagged values set in the class |
| TaggedValueOnFeature | Meta information on Tagged values set on a named feature in the class |
| Typename | The type name as a string |
| ViewModels | A tuple with the ViewModels for this class a members |
See also: OCLOperators
Keep OCL expressions query-only
OCL evaluates information; it must not change data as it runs. Use OCL for queries, constraints, derivations, and expression-based UI behavior. When you need to change data, use EAL (Extended Action Language) rather than placing updates in an OCL expression.
Learn operators in context
For a guided explanation of operator configuration, derivation, and association tools, see Part 2 OCL: Operators. Turnkey session 7: Expressions shows how operators are combined when retrieving and shaping model data.