I've download a trial of Altova UModel and am starting using UML. As a practical beginning I am modelling a personal information manager application, which includes a web bookmark managing.
A Bookmark can belong to many (or no) Tags at once and a Tag can contain many (or no if all the bookmarks it contained were deleted) bookmarks. The relation has to be both-way navigable - a user has to be able to see all Bookmarks with a particular Tags ans all Tags of a Bookmark.
What is the correct UML relation between Bookmark and Tag classes?
As far as I understand UML now, it is an Association (not an Aggregation). But for a 2-way navigable many-to-many relation I can specify ends roles as "memberEnd" or "when navigableOwnedEnd", graphically the connection looks the same in both cases (an arrow) (which as I understand means navigability) but a property appears in the class box in case only when "memberEnd" is used.
How should I specif it in the model If I mean both-way navigable many-to-many relation there?
From UML Superstructure Specification, v2.1.2 section 7.3.3:
memberEnd : Property [2..*]
Each end represents participation of instances of the classifier connected to the end in links of the association. This is ordered association. Subsets Namespace::member.
ownedEnd : Property [*]
The ends that are owned by the association itself. This is an ordered association. Subsets Association::memberEnd,
Classifier::feature, and Namespace::ownedMember.
navigableOwnedEnd : Property [*]
The navigable ends that are owned by the association itself. Subsets Association::ownedEnd.
So if the end is 'owned' by the association, use the ownedEnd/navigableOwnedEnd type, otherwise use the memberEnd type.
Either can be used for a 'both-way navigable many-to-many relation'; if each relation link is a separate instance in your design, it can own the ends (e.g. class A and class B has a reference to a list of pairs of references to related As and Bs), but if the relation link is implicit then it does not own anything (e.g. class A has a list of references to related Bs, class B has a list of references to related As).
Having used UML since the late `90s, you're the first person I've met who cared about the difference!
Related
I'm having a difficulty figuring out where to put the Level attribute. I want the employee to have a variety of skills which have different levels. For that purpose I made a many to many relationship which is implemented as an EmployeeSkill association class.
An example of said class is as follows: An employee named Jack might have a skill of Java which is level beginner, while he could also have a skill of C# which is advanced level. Should the level be saved in the EmployeeSkill association class or in the Skill class? I suspect it should be on the association class.
A property of the association class?
Your narrative with the Employee instance "Jack" being associated with Skill instance "Java" with a level of "beginner" shows that the skill is independent of the level, and the level is related to the combination of a given employee and a given skill.
To model accurately the semantic of this narrative, level should indeed be the property of the association class EmployeeSkill.
Could it be a property of the skills?
It would not be wrong to move level as a property of Skill. But the model would have a different semantic, since every Employee associated with a Skill would share the same level. This means that "Jack" being associated to a skill like "Java" but rather a skill like Elementary Java.
This alternative model makes it also less convenient to desperately search for all employees knowing about Neural network regardless of the level. And it would not be obvious for the system to see that elementary neural network is related to advanced neural network expertise and that Advanced java has little to do with Advanced javascript.
To have the same power of expression, the skills that are related but correspond to a different level would require an explicit association, which would make the model more cumbersome to use and fragile.
Remarks that are unrelated to your question
You seem to repeat the JobCategory's and the Country's attribute within Employee. This is confusing, as the associations already associate the class with another set of each of these attributes. Did you mean the redundant attributes to implement the associations and show the db table layout than the class? If so, you should disambiguate using some custom stereotypes. But I'd rather recommend to remove the redundant items and make a comment at the bottom of the diagram to explain that you implement associations.
I wonder if the multiplicities of the association between Skill and SkillCategory are not inverted (i.e. one category can have several skills and each skill is associated with only one category) ? Or that you meant a many-to-many association if skills can belong to several categories.
In the association class you use the prefix FK to highlight the foreign keys. Use <<FK>> to make it a custom stereotype (see may first remark about using stereotypes).
As qwerty_so pointed out in the comments, you use a nesting connector for the associations with enumerations. This is syntactically acceptable but does not mean what you think. This is meant to deal with namespaces. Just remove that circle-plus symbol to make it a normal association and it will mean what you think.
I have a hard time understanding the concept of association class as explained in UML 2.5 specification. The thing that confuses me the most is the following sentence taken from page 199:
NOTE. Even when all ends of the AssociationClass have isUnique=true, it is possible to have several instances associating the same set of instances of the end Classes.
As noted here: https://issues.omg.org/issues/UMLR-757 this single sentence seems to undermine the usefulness of the concept. At the same time, it makes pretty much every text explaining the concept written before 2.5 version of the UML spec obsolete – see for example the discussion here: UML association class - clarifying
But how can this actually work conceptually? Assuming all ends of the association class have isUnique=true, how can one have more than one instance of association class associating the same set of instances of the end classes, when isUnique property of member ends clearly forbids existence of such links?
I’ve always thought that an association class is just a regular association with extra attributes and/or operations. That interpretation seems to invalid now. So what exactly is association class? How can it have uniqueness independent from that of association member ends? There seem to be some unspoken multiplicity lurking somewhere (uniqueness is irrelevant property without maximum multiplicity higher than 1), but I can't figure out where.
In very short
Indeed, this is not super-clear and would deserve a better explanation. To make it short: it’s just a consequence of the lack of integration in the dual semantics defined for the association class.
Detailed explanation
What is uniqueness for an association
According to UML 2.5.1 section 11.5.3.1, page 197:
When one or more ends of the Association have isUnique=false, it is possible to have several links associating the same set of instances.
We can deduct using logical contraposition that:
If it is not possible to have several links associating the same set of instances, all ends of the association have isUnique=true.
So we'd expect this to apply to an assocation class as well, since the association class is also an association.
Association classes are two distinct things at the same time
According to UML 2.5.1 section 11.5.3.2:
An AssociationClass is both an Association and a Class, and preserves the static and dynamic semantics of both.
So, an association class is not just "an association with extra attributes". If it were that simple, the association class could perfectly be a generalization of an association: the specialized association would just inherit the extra attributes. But this is explicitly prohibited page 199:
An AssociationClass cannot be a generalization of an Association or a
Class.
because any specialization into a class would loose the association semantics, and any specialization into an association would loose the class semantics.
And this duality, is the cause of our issue.
Impact of this duality on instances
According to UML section 11.5.3.2, page 199 (formatting from me):
An instance of an AssociationClass has the characteristics both of a link representing an instantiation of the AssociationClass as a kind of Association, AND of an object representing an instantiation of the AssociationClass as a kind of Class.
If isUnique=true for all association ends, the instances of the association are guaranteed to be unique. Remind however that the association is only about tuples made out of the association ends:
An Association declares that there can be links between instances whose types conform to or implement the associated types. A link is a tuple with one value for each memberEnd of the Association, where each value is an instance whose type conforms to or implements the type at the end.
However, nothing in the specs requires that the class instantiation (object) representing the association instantiation needs to be unique.
Imagine for example, that we have an association class between class A and class B, and a and b are instances of these classes. Imagine that the association ends have isUnique=true. This means that there can be only one tuple (a,b) since the association is guaranteed to be unique.
Let P be a property of the association class, and let (a,b,p1) and (a,b,p2) be two instances of the class in the association-class. The class knows no association-ends: from the point of view of the class, there is no unicity requirement. And from the point of view of the association, we have only one tuple (a,b) , so it's ok as well.
The NOTE just explains that this (unfortunate and ambiguous) situation is possible.
Are there contradictions or inconsistencies?
Formally, there is no contradiction. This is the result of the way the association class is defined: a class and an association at the same time, without further defining the relationship between the respective instances.
But this creates some issues in regard of the semantics of associations having non-unique ends:
When one or more ends of the Association have isUnique=false, it is possible to have several links associating the same set of instances. In such a case, links carry an additional identifier apart from their end values.
More precisely, this makes the the association-class with unique ends useless, since the same result can be achieved with non-unique ends:
for a simple association with non-unique ends, you can have duplicates, i.e. several links associating the same instances of the association ends that are distinguished with an additional identifier.
for association classes with unique ends, according to the note you can have duplicates, i.e. several objects (class instances) corresponding to a link made of unique association ends (association instances).
for an association-class with non-unique ends, you could have duplicates, i.e. several object instances correspond to the same set of member ends. It makes no difference if you interpret this as several links associating the same instances of the association ends, each associated with a single object, or, if you interpret this as one link associating a unique set of instances of the association end, that would be each be associated to mutliple object instances.
IMHO, this is unfortunate:
it does not match our mental model in which an association class with all association ends having isUnique=true should have a unique object instance of the class for a unique combination association ends. This clearly goes against the principle of least astonishment: I started with deny and it took me a while to accept this, since it was so terribly different from the traditional ways to implement association classes.
two different models, one with unique association ends, and one without could in fact express the same situation.
A simple solution to this issue would be to require a unique class instance (object corresponding to the association class) to correspond to a link that uniquely associates association ends. In that way, unique association ends would imply unique association object, without requiring other changes to the UML specs.
The diagram below is based on these facts:
A citizen may make a claim / reclamation on pollutions
An administrator will send the reclamation by type to the appropriate agency / organization, and it will respond with a diagnostic report
This report will be sent by the administrator, to another organization, for the estimation of the solution (costs, etc.).
My problem is to how can I show that Admin will send the claim to an agency? Do I have to make an association between Organization and Reclamation?
If your requirements include that the communication between the Admin and the Organization is to be documented, then you need to take care of this in your class diagram. And, in fact, you already have an item for this: the NotifyOrganization association class. Notice, however, that it's not a good idea to use the questionable UML concept of an association class, which does not have a clear semantics and is confusing. This seems to be confirmed by your flawed modeling of the multiplicities of the NotifyOrganization association, which must not be one-to-one (or 1 to 0..1), but rather many-to-many (* to *).
So, better replace the NotifyOrganization association class with an ordinary class (possibly with an improved name such as Notification) and attach it to Admin and Organization with two many-to-one associations such that each notification is linked to exactly one admin and one Organization.
Notice that a notification represents a (communication/message) event, so Notification represents an event type. It*s quite common in business information models to have both object types and event types, both as classes in a UML class diagram.
...how can I show that Admin will send the claim to an agency?
UML Class Diagram is not show-everything document. In order to document that admin will send claim to agency you can document it effectively using UML Sequence Diagram or some other behavior diagram
...do I have to make an association between Organization and Reclamation?
No, if the agency does not need to care about who exactly delivered the claim or what's the person's role (e.g. "admin"), then you don' have to add it to the class model (and show some association) at all
...diagram...based on these facts...
In order to describe the overall process in a one-page style picture, you (and your business partners) can find very useful the Business Process Model and Notation (BPMN) graphical language in addition to UML
When creating references between ECore entities there is the possibility to mark a reference as "containment".
Can somebody explain me in easy words what's the difference between a plain reference and a containment? The definitions and explainations I found so far didn't do the trick for me.
Reference: A reference is a plain "A knows B" relation. Separate references do not influence each other. A can know B and B can know C. Hence, if you have A, you can go to C over B. If you remove the reference, A, B and C will still exist, they just don't know each other any more.
Containment: A containment is the "A has B" kind of relation. Usually used for lists, e.g. "A has multiple B". ECore/EMF can then perform atomic commands on such collections, such as move all objects from one containment to another. It can also enforce constraints, such as a minimum amount of contained objects or a maximum amount of contained objects, or ensuring that the contained object is not contained in any other containment.
Example:
Assume you have an object called ShoppingCart with a reference called Customer and a containment called OrderedProducts. The OrderedProducts has a reference to a Product.
What does this model tell you?
You can assign a Customer to the ShoppingCart. If you remove the Customer from the ShoppingCart, the Customer object itself will still exist (e.g. in the database)
The OrderedProduct objects need a ShoppingCart to exist. If you remove one from the ShoppingCart, it will cease to exist.
Each OrderedProduct has a reference to an existing Product in the database. If you remove one of the OrderedProducts from the ShoppingCart, the Product in the database will still be there - just the order of that product for that specific customer is gone
Reference : can be like association in UML
Containment : is the composition relation from UML
Also this can be seen in the EMF diagram, if you set the containment to true, then the line will have a filled side, showing the containment/composition. This is what I have observed.
I am trying to figure out if it is correct to put in the fields of the particular class a reference to an object/collection that this class is related with.
Let's say I have a class University that aggregates many instances of Student class. When I put on my diagram both classes, I add the relationship of aggregation between them.
And now the question: Can I add in University a field like 'students : Student[]'? Do I have to? Or maybe I must not?
Thanks in advance,
Piotr
Attributes own association properties in UML 2. This is new and it is not always easy to understand the concept. I didn't understand why we need an attribute till I have seen the following demo.
The golden rule is that attributes should be created in order to save association information in the model. I have found an interesting teaching session on association and aggregation in UML 2.
It is a 2m30s flash demo
http://www.download-omondo.com/AggregationAndComposition.swf
The above association creation is for expert level, I mean modeler who wants clean design and clean model as well as perfect metamodel. If you don't care then just create an association at model level and it would do the job but the code generation will not be done. It means that mappings for database will needed to be created at code level later by developers.
To understand the difference between both kinds of associations see this other flash demo at: http://www.download-omondo.com/association.swf
It depends on what you are doing with your model, but generally, you shouldn't have to use an attribute for this, you can use an association instead. Associations are able to contain more information that attributes, such as whether they are composite or shared, which ends are navigable, multiplicity on each end, named endpoints, etc.
One benefit of associations is that one association can actually represent a property on each class it is connected to, while an attribute only represents a property on the class it belongs to.
So, in your case, University has Students, so you'd draw an association from University to Student, naming each endpoint and setting the multiplicity correctly.
Now when you go to generate code from your model, just be sure to take into account associations as well as attributes. This way you won't need to add the properties both as attributes and associations, just as associations. I have a few diagrams I've tested this approach with, generating sql and php code from the xmi via an xsl transform, and it works quite well... if you'd like more details I can dig it up.
There are 3 types of relations : association, aggregation and composition. composition is a specialization of aggregation, aggregation is a specialization of association.
Using one or the other depends on the phase of your analysis. You could just use association in first draft and then refine it later to aggregation or composition, the difference with aggregation is that a student had no meaning out of one University Universe. If there are several universities instances, it would be aggregation rather.
If you use relations, there's no need to and you shouldn't add students[] because it is redundant. And if you do that you lose the semantics of the 3 types of relations.